References sql что это

Для связи между таблицами применяются внешние ключи. Внешний ключ устанавливается для столбца из зависимой, подчиненной таблицы (referencing table), и указывает на один из столбцов из главной таблицы (referenced table). Как правило, внешний ключ указывает на первичный ключ из связанной главной таблицы.

Общий синтаксис установки внешнего ключа на уровне столбца:

Чтобы установить связь между таблицами, после ключевого слова REFERENCES указывается имя связанной таблицы и далее в скобках имя столбца из этой таблицы, на который будет указывать внешний ключ. После выражения REFERENCES может идти выражение ON DELETE и ON UPDATE , которые уточняют поведение при удалении или обновлении данных.

Общий синтаксис установки внешнего ключа на уровне таблицы:

Например, определим две таблицы и свяжем их посредством внешнего ключа:

Здесь определены таблицы Customers и Orders. Customers является главной и представляет клиента. Orders является зависимой и представляет заказ, сделанный клиентом. Эта таблица через столбец CustomerId связана с таблицей Customers и ее столбцом Id. То есть столбец CustomerId является внешним ключом, который указывает на столбец Id из таблицы Customers.

Определение внешнего ключа на уровне таблицы выглядело бы следующим образом:

ON DELETE и ON UPDATE

С помощью выражений ON DELETE и ON UPDATE можно установить действия, которые выполняются соответственно при удалении и изменении связанной строки из главной таблицы. Для установки подобного действия можно использовать следующие опции:

CASCADE : автоматически удаляет или изменяет строки из зависимой таблицы при удалении или изменении связанных строк в главной таблице.

RESTRICT : предотвращает какие-либо действия в зависимой таблице при удалении или изменении связанных строк в главной таблице. То есть фактически какие-либо действия отсутствуют.

NO ACTION : действие по умолчанию, предотвращает какие-либо действия в зависимой таблице при удалении или изменении связанных строк в главной таблице. И генерирует ошибку. В отличие от RESTRICT выполняет отложенную проверку на связанность между таблицами.

SET NULL : при удалении связанной строки из главной таблицы устанавливает для столбца внешнего ключа значение NULL.

SET DEFAULT : при удалении связанной строки из главной таблицы устанавливает для столбца внешнего ключа значение по умолчанию, которое задается с помощью атрибуты DEFAULT. Если для столбца не задано значение по умолчанию, то в качестве него применяется значение NULL.

Каскадное удаление

По умолчанию, если на строку из главной таблицы по внешнему ключу ссылается какая-либо строка из зависимой таблицы, то мы не сможем удалить эту строку из главной таблицы. Вначале нам необходимо будет удалить все связанные строки из зависимой таблицы. И если при удалении строки из главной таблицы необходимо, чтобы были удалены все связанные строки из зависимой таблицы, то применяется каскадное удаление, то есть опция CASCADE :

Аналогично работает выражение ON UPDATE CASCADE . При изменении значения первичного ключа автоматически изменится значение связанного с ним внешнего ключа. Но так как первичные ключи, как правило, изменяются очень редко, да и с принципе не рекомендуется использовать в качестве первичных ключей столбцы с изменяемыми значениями, то на практике выражение ON UPDATE используется редко.

Установка NULL

При установки для внешнего ключа опции SET NULL необходимо, чтобы столбец внешнего ключа допускал значение NULL:

Установка значения по умолчанию

Если для столца значение по умолчанию не задано через параметр DEFAULT, то в качестве такового используется значение NULL (если столбец допускает NULL).

constraint

Use a constraint to define an integrity constraint— a rule that restricts the values in a database. Oracle Database lets you create six types of constraints and lets you declare them in two ways.

The six types of integrity constraint are described briefly here and more fully in «Semantics»:

A NOT NULL constraint prohibits a database value from being null.

A unique constraint prohibits multiple rows from having the same value in the same column or combination of columns but allows some values to be null.

A primary key constraint combines a NOT NULL constraint and a unique constraint in a single declaration. It prohibits multiple rows from having the same value in the same column or combination of columns and prohibits values from being null.

A foreign key constraint requires values in one table to match values in another table.

A check constraint requires a value in the database to comply with a specified condition.

A REF column by definition references an object in another object type or in a relational table. A REF constraint lets you further describe the relationship between the REF column and the object it references.

You can define constraints syntactically in two ways:

As part of the definition of an individual column or attribute. This is called inline specification.

As part of the table definition. This is called out-of-line specification.

NOT NULL constraints must be declared inline. All other constraints can be declared either inline or out of line.

Constraint clauses can appear in the following statements:

View Constraints Oracle Database does not enforce view constraints. However, you can enforce constraints on views through constraints on base tables.

You can specify only unique, primary key, and foreign key constraints on views, and they are supported only in DISABLE NOVALIDATE mode. You cannot define view constraints on attributes of an object column.

«View Constraints» for additional information on view constraints and «DISABLE Clause» for information on DISABLE NOVALIDATE mode

You must have the privileges necessary to issue the statement in which you are defining the constraint.

To create a foreign key constraint, in addition, the parent table or view must be in your own schema or you must have the REFERENCES privilege on the columns of the referenced key in the parent table or view.

( global_partitioned_index ::= , local_partitioned_index ::= —part of CREATE INDEX , index_attributes ::= . The INDEXTYPE IS . clause is not valid when defining a constraint.)

( physical_attributes_clause ::= , logging_clause ::= , key_compression ::= —all part of CREATE INDEX , parallel_clause : not supported in using_index_clause )

This section describes the semantics of constraint . For additional information, refer to the SQL statement in which you define or redefine a constraint for a table or view.

Oracle Database does not support constraints on columns or attributes whose type is a user-defined object, nested table, VARRAY , REF , or LOB, with two exceptions:

NOT NULL constraints are supported for a column or attribute whose type is user-defined object, VARRAY , REF , or LOB.

NOT NULL , foreign key, and REF constraints are supported on a column of type REF .

CONSTRAINT constraint_name Specify a name for the constraint. If you omit this identifier, then Oracle Database generates a name with the form SYS_C n . Oracle stores the name and the definition of the integrity constraint in the USER_ , ALL_ , and DBA_CONSTRAINTS data dictionary views (in the CONSTRAINT_NAME and SEARCH_CONDITION columns, respectively).

Oracle Database Reference for information on the data dictionary views

NOT NULL Constraints

A NOT NULL constraint prohibits a column from containing nulls. The NULL keyword by itself does not actually define an integrity constraint, but you can specify it to explicitly permit a column to contain nulls. You must define NOT NULL and NULL using inline specification. If you specify neither NOT NULL nor NULL , then the default is NULL .

NOT NULL constraints are the only constraints you can specify inline on XMLType and VARRAY columns.

To satisfy a NOT NULL constraint, every row in the table must contain a value for the column.

Oracle Database does not index table rows in which all key columns are null except in the case of bitmap indexes. Therefore, if you want an index on all rows of a table, then you must either specify NOT NULL constraints for at least one of the index key columns or create a bitmap index.

Restrictions on NOT NULL Constraints NOT NULL constraints are subject to the following restrictions:

You cannot specify NULL or NOT NULL in a view constraint.

You cannot specify NULL or NOT NULL for an attribute of an object. Instead, use a CHECK constraint with the IS [ NOT ] NULL condition.

A unique constraint designates a column as a unique key. A composite unique key designates a combination of columns as the unique key. When you define a unique constraint inline, you need only the UNIQUE keyword. When you define a unique constraint out of line, you must also specify one or more columns. You must define a composite unique key out of line.

To satisfy a unique constraint, no two rows in the table can have the same value for the unique key. However, the unique key made up of a single column can contain nulls. To satisfy a composite unique key, no two rows in the table or view can have the same combination of values in the key columns. Any row that contains nulls in all key columns automatically satisfies the constraint. However, two rows that contain nulls for one or more key columns and the same combination of values for the other key columns violate the constraint.

When you specify a unique constraint on one or more columns, Oracle implicitly creates an index on the unique key. If you are defining uniqueness for purposes of query performance, then Oracle recommends that you instead create the unique index explicitly using a CREATE UNIQUE INDEX statement. You can also use the CREATE UNIQUE INDEX statement to create a unique function-based index that defines a conditional unique constraint. See «Using a Function-based Index to Define Conditional Uniqueness: Example» for more information.

Restrictions on Unique Constraints Unique constraints are subject to the following restrictions:

None of the columns in the unique key can be of LOB, LONG , LONG RAW , VARRAY , NESTED TABLE , OBJECT , REF , TIMESTAMP WITH TIME ZONE, or user-defined type. However, the unique key can contain a column of TIMESTAMP WITH LOCAL TIME ZONE .

A composite unique key cannot have more than 32 columns.

You cannot designate the same column or combination of columns as both a primary key and a unique key.

You cannot specify a unique key when creating a subview in an inheritance hierarchy. The unique key can be specified only for the top-level (root) view.

Primary Key Constraints

A primary key constraint designates a column as the primary key of a table or view. A composite primary key designates a combination of columns as the primary key. When you define a primary key constraint inline, you need only the PRIMARY KEY keywords. When you define a primary key constraint out of line, you must also specify one or more columns. You must define a composite primary key out of line.

To satisfy a primary key constraint:

No primary key value can appear in more than one row in the table.

No column that is part of the primary key can contain a null.

When you create a primary key constraint:

Oracle Database uses an existing index if it contains a unique set of values before enforcing the primary key constraint. The existing index can be defined as unique or nonunique. When a DML operation is performed, the primary key constraint is enforced using this existing index.

If no existing index can be used, then Oracle Database generates a unique index.

When you drop a primary key constraint:

If the primary key was created using an existing index, then the index is not dropped.

If the primary key was created using a system-generated index, then the index is dropped.

Restrictions on Primary Key Constraints Primary constraints are subject to the following restrictions:

A table or view can have only one primary key.

None of the columns in the primary key can be LOB, LONG , LONG RAW , VARRAY , NESTED TABLE , BFILE , REF , TIMESTAMP WITH TIME ZONE , or user-defined type. However, the primary key can contain a column of TIMESTAMP WITH LOCAL TIME ZONE .

The size of the primary key cannot exceed approximately one database block.

A composite primary key cannot have more than 32 columns.

You cannot designate the same column or combination of columns as both a primary key and a unique key.

You cannot specify a primary key when creating a subview in an inheritance hierarchy. The primary key can be specified only for the top-level (root) view.

Foreign Key Constraints

A foreign key constraint (also called a referential integrity constraint ) designates a column as the foreign key and establishes a relationship between that foreign key and a specified primary or unique key, called the referenced key . A composite foreign key designates a combination of columns as the foreign key.

The table or view containing the foreign key is called the child object, and the table or view containing the referenced key is called the parent object. The foreign key and the referenced key can be in the same table or view. In this case, the parent and child tables are the same. If you identify only the parent table or view and omit the column name, then the foreign key automatically references the primary key of the parent table or view. The corresponding column or columns of the foreign key and the referenced key must match in order and data type.

You can define a foreign key constraint on a single key column either inline or out of line. You must specify a composite foreign key and a foreign key on an attribute out of line.

To satisfy a composite foreign key constraint, the composite foreign key must refer to a composite unique key or a composite primary key in the parent table or view, or the value of at least one of the columns of the foreign key must be null.

You can designate the same column or combination of columns as both a foreign key and a primary or unique key. You can also designate the same column or combination of columns as both a foreign key and a cluster key.

You can define multiple foreign keys in a table or view. Also, a single column can be part of more than one foreign key.

Restrictions on Foreign Key Constraints Foreign key constraints are subject to the following restrictions:

None of the columns in the foreign key can be of LOB, LONG , LONG RAW , VARRAY , NESTED TABLE , BFILE , REF , TIMESTAMP WITH TIME ZONE , or user-defined type. However, the primary key can contain a column of TIMESTAMP WITH LOCAL TIME ZONE .

The referenced unique or primary key constraint on the parent table or view must already be defined.

A composite foreign key cannot have more than 32 columns.

The child and parent tables must be on the same database. To enable referential integrity constraints across nodes of a distributed database, you must use database triggers. See CREATE TRIGGER.

If either the child or parent object is a view, then the constraint is subject to all restrictions on view constraints. See «View Constraints».

You cannot define a foreign key constraint in a CREATE TABLE statement that contains an AS subquery clause. Instead, you must create the table without the constraint and then add it later with an ALTER TABLE statement.

When a table has a foreign key, and the parent of the foreign key is an index-organized table, a session that updates a row that contains the foreign key can hang when another session is updating a non-key column in the parent table.

references_clause Foreign key constraints use the references_clause syntax. When you specify a foreign key constraint inline, you need only the references_clause . When you specify a foreign key constraint out of line, you must also specify the FOREIGN KEY keywords and one or more columns.

ON DELETE Clause The ON DELETE clause lets you determine how Oracle Database automatically maintains referential integrity if you remove a referenced primary or unique key value. If you omit this clause, then Oracle does not allow you to delete referenced key values in the parent table that have dependent rows in the child table.

Specify CASCADE if you want Oracle to remove dependent foreign key values.

Specify SET NULL if you want Oracle to convert dependent foreign key values to NULL . You cannot specify this clause for a virtual column, because the values in a virtual column cannot be updated directly. Rather, the values from which the virtual column are derived must be updated.

Restriction on ON DELETE You cannot specify this clause for a view constraint.

A check constraint lets you specify a condition that each row in the table must satisfy. To satisfy the constraint, each row in the table must make the condition either TRUE or unknown (due to a null). When Oracle evaluates a check constraint condition for a particular row, any column names in the condition refer to the column values in that row.

The syntax for inline and out-of-line specification of check constraints is the same. However, inline specification can refer only to the column (or the attributes of the column if it is an object column) currently being defined, whereas out-of-line specification can refer to multiple columns or attributes.

Oracle does not verify that conditions of check constraints are not mutually exclusive. Therefore, if you create multiple check constraints for a column, design them carefully so their purposes do not conflict. Do not assume any particular order of evaluation of the conditions.

Chapter 7, «Conditions» for additional information and syntax

Restrictions on Check Constraints Check constraints are subject to the following restrictions:

You cannot specify a check constraint for a view. However, you can define the view using the WITH CHECK OPTION clause, which is equivalent to specifying a check constraint for the view.

The condition of a check constraint can refer to any column in the table, but it cannot refer to columns of other tables.

Conditions of check constraints cannot contain the following constructs:

Subqueries and scalar subquery expressions

Calls to the functions that are not deterministic ( CURRENT_DATE , CURRENT_TIMESTAMP , DBTIMEZONE , LOCALTIMESTAMP , SESSIONTIMEZONE , SYSDATE , SYSTIMESTAMP , UID , USER , and USERENV )

Calls to user-defined functions

Dereferencing of REF columns (for example, using the DEREF function)

Nested table columns or attributes

The pseudocolumns CURRVAL , NEXTVAL , LEVEL , or ROWNUM

Date constants that are not fully specified

REF constraints let you describe the relationship between a column of type REF and the object it references.

ref_constraint REF constraints use the ref_constraint syntax. You define a REF constraint either inline or out of line. Out-of-line specification requires you to specify the REF column or attribute you are further describing.

For ref_column , specify the name of a REF column of an object or relational table.

For ref_attribute , specify an embedded REF attribute within an object column of a relational table.

Both inline and out-of-line specification let you define a scope constraint, a rowid constraint, or a referential integrity constraint on a REF column.

If the scope table or referenced table of the REF column has a primary-key-based object identifier, then the REF column is a user-defined REF column .

SCOPE REF Constraints

In a table with a REF column, each REF value in the column can conceivably reference a row in a different object table. The SCOPE clause restricts the scope of references to a single table, scope_table . The values in the REF column or attribute point to objects in scope_table , in which object instances of the same type as the REF column are stored.

Specify the SCOPE clause to restrict the scope of references in the REF column to a single table. For you to specify this clause, scope_table must be in your own schema or you must have SELECT privileges on scope_table or SELECT ANY TABLE system privileges. You can specify only one scope table for each REF column.

Restrictions on Scope Constraints Scope constraints are subject to the following restrictions:

You cannot add a scope constraint to an existing column unless the table is empty.

You cannot specify a scope constraint for the REF elements of a VARRAY column.

You must specify this clause if you specify AS subquery and the subquery returns user-defined REF data types.

You cannot subsequently drop a scope constraint from a REF column.

Rowid REF Constraints

Specify WITH ROWID to store the rowid along with the REF value in ref_column or ref_attribute . Storing the rowid with the REF value can improve the performance of dereferencing operations, but will also use more space. Default storage of REF values is without rowids.

The function DEREF for an example of dereferencing

Restrictions on Rowid Constraints Rowid constraints are subject to the following restrictions:

You cannot define a rowid constraint for the REF elements of a VARRAY column.

You cannot subsequently drop a rowid constraint from a REF column.

If the REF column or attribute is scoped, then this clause is ignored and the rowid is not stored with the REF value.

Referential Integrity Constraints on REF Columns

The references_clause of the ref_constraint syntax lets you define a foreign key constraint on the REF column. This clause also implicitly restricts the scope of the REF column or attribute to the referenced table. However, whereas a foreign key constraint on a non- REF column references an actual column in the parent table, a foreign key constraint on a REF column references the implicit object identifier column of the parent table.

If you do not specify a constraint name, then Oracle generates a system name for the constraint of the form SYS_C n .

If you add a referential integrity constraint to an existing REF column that is already scoped, then the referenced table must be the same as the scope table of the REF column. If you later drop the referential integrity constraint, then the REF column will remain scoped to the referenced table.

As is the case for foreign key constraints on other types of columns, you can use the references_clause alone for inline declaration. For out-of-line declaration you must also specify the FOREIGN KEY keywords plus one or more REF columns or attributes.

Restrictions on Foreign Key Constraints on REF Columns Foreign key constraints on REF columns have the following additional restrictions:

Oracle implicitly adds a scope constraint when you add a referential integrity constraint to an existing unscoped REF column. Therefore, all the restrictions that apply for scope constraints also apply in this case.

You cannot specify a column after the object name in the references_clause .

Specifying Constraint State

As part of constraint definition, you can specify how and when Oracle should enforce the constraint.

constraint_state You can use the constraint_state with both inline and out-of-line specification. Specify the clauses of constraint_state in the order shown, from top to bottom, and do not specify any clause more than once.

DEFERRABLE Cla use The DEFERRABLE and NOT DEFERRABLE parameters indicate whether or not, in subsequent transactions, constraint checking can be deferred until the end of the transaction using the SET CONSTRAINT ( S ) statement. If you omit this clause, then the default is NOT DEFERRABLE .

Specify NOT DEFERRABLE to indicate that in subsequent transactions you cannot use the SET CONSTRAINT [ S ] clause to defer checking of this constraint until the transaction is committed. The checking of a NOT DEFERRABLE constraint can never be deferred to the end of the transaction.

If you declare a new constraint NOT DEFERRABLE , then it must be valid at the time the CREATE TABLE or ALTER TABLE statement is committed or the statement will fail.

Specify DEFERRABLE to indicate that in subsequent transactions you can use the SET CONSTRAINT [ S ] clause to defer checking of this constraint until a COMMIT statement is submitted. If the constraint check fails, then the database returns an error and the transaction is not committed. This setting in effect lets you disable the constraint temporarily while making changes to the database that might violate the constraint until all the changes are complete.

The optimizer does not consider indexes on deferrable constraints as usable.

You cannot alter the deferrability of a constraint. Whether you specify either of these parameters, or make the constraint NOT DEFERRABLE implicitly by specifying neither of them, you cannot specify this clause in an ALTER TABLE statement. You must drop the constraint and re-create it.

SET CONSTRAINT[S] for information on setting constraint checking for a transaction

Restriction on [NOT] DEFERRABLE You cannot specify either of these parameters for a view constraint.

INITIALLY Clause The INITIALLY clause establishes the default checking behavior for constraints that are DEFERRABLE . The INITIALLY setting can be overridden by a SET CONSTRAINT ( S ) statement in a subsequent transaction.

Specify INITIALLY IMMEDIATE to indicate that Oracle should check this constraint at the end of each subsequent SQL statement. If you do not specify INITIALLY at all, then the default is INITIALLY IMMEDIATE .

If you declare a new constraint INITIALLY IMMEDIATE , then it must be valid at the time the CREATE TABLE or ALTER TABLE statement is committed or the statement will fail.

Specify INITIALLY DEFERRED to indicate that Oracle should check this constraint at the end of subsequent transactions.

This clause is not valid if you have declared the constraint to be NOT DEFERRABLE , because a NOT DEFERRABLE constraint is automatically INITIALLY IMMEDIATE and cannot ever be INITIALLY DEFERRED .

VALIDATE | NOVALIDATE The behavior of VALIDATE and NOVALIDATE depends on whether the constraint is enabled or disabled, either explicitly or by default. Therefore, the VALIDATE and NOVALIDATE keywords are described in the context of «ENABLE Clause» and «DISABLE Clause».

Note on Foreign Key Constraints in NOVALIDATE Mode When a foreign key constraint is in NOVALIDATE mode, if existing data in the table does not comply with the constraint and the QUERY_REWRITE_INTEGRITY parameter is not set to ENFORCED , then the optimizer may use join elimination during queries on the table. In this case, a query may return table rows with noncompliant foreign key values even if the query contains a join condition that should filter out those rows.

ENABLE Clause Specify ENABLE if you want the constraint to be applied to the data in the table.

If you enable a unique or primary key constraint, and if no index exists on the key, then Oracle Database creates a unique index. Unless you specify KEEP INDEX when subsequently disabling the constraint, this index is dropped and the database rebuilds the index every time the constraint is reenabled.

You can also avoid rebuilding the index and eliminate redundant indexes by creating new primary key and unique constraints initially disabled. Then create (or use existing) nonunique indexes to enforce the constraint. Oracle does not drop a nonunique index when the constraint is disabled, so subsequent ENABLE operations are facilitated.

ENABLE VALIDATE specifies that all old and new data also complies with the constraint. An enabled validated constraint guarantees that all data is and will continue to be valid.

If any row in the table violates the integrity constraint, then the constraint remains disabled and Oracle returns an error. If all rows comply with the constraint, then Oracle enables the constraint. Subsequently, if new data violates the constraint, then Oracle does not execute the statement and returns an error indicating the integrity constraint violation.

If you place a primary key constraint in ENABLE VALIDATE mode, then the validation process will verify that the primary key columns contain no nulls. To avoid this overhead, mark each column in the primary key NOT NULL before entering data into the column and before enabling the primary key constraint of the table.

ENABLE NOVALIDATE ensures that all new DML operations on the constrained data comply with the constraint. This clause does not ensure that existing data in the table complies with the constraint.

If you specify neither VALIDATE nor NOVALIDATE , then the default is VALIDATE .

If you change the state of any single constraint from ENABLE NOVALIDATE to ENABLE VALIDATE , then the operation can be performed in parallel, and does not block reads, writes, or other DDL operations.

Restriction on the ENABLE Clause You cannot enable a foreign key that references a disabled unique or primary key.

DISABLE Clause Specify DISABLE to disable the integrity constraint. Disabled integrity constraints appear in the data dictionary along with enabled constraints. If you do not specify this clause when creating a constraint, then Oracle automatically enables the constraint.

DISABLE VALIDATE disables the constraint and drops the index on the constraint, but keeps the constraint valid. This feature is most useful in data warehousing situations, because it lets you load large amounts of data while also saving space by not having an index. This setting lets you load data from a nonpartitioned table into a partitioned table using the exchange_partition_subpart clause of the ALTER TABLE statement or using SQL*Loader. All other modifications to the table (inserts, updates, and deletes) by other SQL statements are disallowed.

Oracle Database Data Warehousing Guide for more information on using this setting

DISABLE NOVALIDATE signifies that Oracle makes no effort to maintain the constraint (because it is disabled) and cannot guarantee that the constraint is true (because it is not being validated).

You cannot drop a table whose primary key is being referenced by a foreign key even if the foreign key constraint is in DISABLE NOVALIDATE state. Further, the optimizer can use constraints in DISABLE NOVALIDATE state.

Oracle Database Performance Tuning Guide for information on when to use this setting

If you specify neither VALIDATE nor NOVALIDATE , then the default is NOVALIDATE .

If you disable a unique or primary key constraint that is using a unique index, then Oracle drops the unique index. Refer to the CREATE TABLE enable_disable_clause for additional notes and restrictions.

RELY Clause The RELY and NORELY parameters specify whether a constraint in NOVALIDATE mode is to be taken into account for query rewrite. Specify RELY to activate a constraint in NOVALIDATE mode for query rewrite in an unenforced query rewrite integrity mode. The constraint is in NOVALIDATE mode, so Oracle does not enforce it. The default is NORELY .

Unenforced constraints are generally useful only with materialized views and query rewrite. Depending on the QUERY_REWRITE_INTEGRITY mode, query rewrite can use only constraints that are in VALIDATE mode, or that are in NOVALIDATE mode with the RELY parameter set, to determine join information.

Restriction on the RELY Clause You cannot set a nondeferrable NOT NULL constraint to RELY .

Oracle Database Data Warehousing Guide for more information on materialized views and query rewrite

Using Indexes to Enforce Constraints

When defining the state of a unique or primary key constraint, you can specify an index for Oracle to use to enforce the constraint, or you can instruct Oracle to create the index used to enforce the constraint.

using_index_clause You can specify the using_index_clause only when enabling unique or primary key constraints. You can specify the clauses of the using_index_clause in any order, but you can specify each clause only once.

If you specify schema . index , then Oracle attempts to enforce the constraint using the specified index. If Oracle cannot find the index or cannot use the index to enforce the constraint, then Oracle returns an error.

If you specify the create_index_statement , then Oracle attempts to create the index and use it to enforce the constraint. If Oracle cannot create the index or cannot use the index to enforce the constraint, then Oracle returns an error.

If you neither specify an existing index nor create a new index, then Oracle creates the index. In this case:

The index receives the same name as the constraint.

If table is partitioned, then you can specify a locally or globally partitioned index for the unique or primary key constraint.

Restrictions on the using_index_clause The following restrictions apply to the using_index_clause :

You cannot specify this clause for a view constraint.

You cannot specify this clause for a NOT NULL , foreign key, or check constraint.

You cannot specify an index ( schema.index ) or create an index ( create_index_statement ) when enabling the primary key of an index-organized table.

You cannot specify the parallel_clause of index_attributes .

The INDEXTYPE IS . clause of index_properties is not valid in the definition of a constraint.

CREATE INDEX for a description of index_attributes , the global_partitioned_index and local_partitioned_index clauses, and for a description of NOSORT and the logging_clause in relation to indexes

Handling Constraint Exceptions

When defining the state of a constraint, you can specify a table into which Oracle places the rowids of all rows violating the constraint.

exceptions_clause Use the exceptions_clause syntax to define exception handling. If you omit schema , then Oracle assumes the exceptions table is in your own schema. If you omit this clause altogether, then Oracle assumes that the table is named EXCEPTIONS . The EXCEPTIONS table or the table you specify must exist on your local database.

You can create the EXCEPTIONS table using one of these scripts:

UTLEXCPT.SQL uses physical rowids. Therefore it can accommodate rows from conventional tables but not from index-organized tables. (See the Note that follows.)

UTLEXPT1.SQL uses universal rowids, so it can accommodate rows from both conventional and index-organized tables.

If you create your own exceptions table, then it must follow the format prescribed by one of these two scripts.

If you are collecting exceptions from index-organized tables based on primary keys (rather than universal rowids), then you must create a separate exceptions table for each index-organized table to accommodate its primary-key storage. You create multiple exceptions tables with different names by modifying and resubmitting the script.

Restrictions on the exceptions_clause The following restrictions apply to the exceptions_clause :

You cannot specify this clause for a view constraint.

You cannot specify this clause in a CREATE TABLE statement, because no rowids exist until after the successful completion of the statement.

The DBMS_IOT package in Oracle Database PL/SQL Packages and Types Reference for information on the SQL scripts

Oracle Database Performance Tuning Guide for information on eliminating migrated and chained rows

Data warehousing applications recognize multidimensional data in the Oracle Database by identifying referential integrity constraints in the relational schema. These constraints represent primary and foreign key relationships among tables. By querying the Oracle Database data dictionary, applications can recognize such constraints and therefore recognize the multidimensional data in the database. For schema complexity or security reasons, you might want to define views on fact and dimension tables. Oracle Database provides the ability to constrain these views. By allowing constraint definitions between views, you can propagate base table constraints to the views, thereby allowing applications to recognize multidimensional data even in the restricted environment provided by the view.

Oracle does not enforce view constraints. However, operations on views are subject to the integrity constraints defined on the underlying base tables. This means that you can enforce constraints on views through constraints on base tables.

Notes on View Constraints View constraints are a subset of table constraints and are subject to the following restrictions:

You can specify only unique, primary key, and foreign key constraints on views. However, you can define the view using the WITH CHECK OPTION clause, which is equivalent to specifying a check constraint for the view.

View constraints are supported only in DISABLE NOVALIDATE mode. You cannot specify any other mode. You must specify the keyword DISABLE when you declare the view constraint. You need not specify NOVALIDATE explicitly, as it is the default.

The RELY and NORELY parameters are optional. View constraints, because they are unenforced, are usually specified with the RELY parameter to make them more useful. The RELY or NORELY keyword must precede the DISABLE keyword. Refer to «RELY Clause» for more information.

Because view constraints are not enforced directly, you cannot specify INITIALLY DEFERRED or DEFERRABLE .

You cannot specify the using_index_clause , the exceptions_clause clause, or the ON DELETE clause of the references_clause .

You cannot define view constraints on attributes of an object column.

Unique Key Example The following statement is a variation of the statement that created the sample table sh.promotions . It defines inline and implicitly enables a unique key on the promo_id column (other constraints are not shown):

The constraint promo_id_u identifies the promo_id column as a unique key. This constraint ensures that no two promotions in the table have the same ID. However, the constraint does allow promotions without identifiers.

Alternatively, you can define and enable this constraint out of line:

The preceding statement also contains the using_index_clause , which specifies storage characteristics for the index that Oracle creates to enable the constraint.

Composite Unique Key Example The following statement defines and enables a composite unique key on the combination of the warehouse_id and warehouse_name columns of the oe.warehouses table:

The wh_unq constraint ensures that the same combination of warehouse_id and warehouse_name values does not appear in the table more than once.

The ADD CONSTRAINT clause also specifies other properties of the constraint:

The USING INDEX clause specifies storage characteristics for the index Oracle creates to enable the constraint.

The EXCEPTIONS INTO clause causes Oracle to write to the wrong_id table information about any rows currently in the warehouses table that violate the constraint. If the wrong_id exceptions table does not already exist, then this statement will fail.

Primary Key Example The following statement is a variation of the statement that created the sample table hr.locations . It creates the locations_demo table and defines and enables a primary key on the location_id column (other constraints from the hr.locations table are omitted):

The loc_id_pk constraint, specified inline, identifies the location_id column as the primary key of the locations_demo table. This constraint ensures that no two locations in the table have the same location number and that no location identifier is NULL .

Alternatively, you can define and enable this constraint out of line:

NOT NULL Example The following statement alters the locations_demo table (created in «Primary Key Example») to define and enable a NOT NULL constraint on the country_id column:

The constraint country_nn ensures that no location in the table has a null country_id .

Composite Primary Key Example The following statement defines a composite primary key on the combination of the prod_id and cust_id columns of the sample table sh.sales :

This constraint identifies the combination of the prod_id and cust_id columns as the primary key of the sales table. The constraint ensures that no two rows in the table have the same combination of values for the prod_id column and cust_id columns.

The constraint clause ( PRIMARY KEY ) also specifies the following properties of the constraint:

The constraint definition does not include a constraint name, so Oracle generates a name for the constraint.

The DISABLE clause causes Oracle to define the constraint but not enable it.

Foreign Key Constraint Example The following statement creates the dept_20 table and defines and enables a foreign key on the department_id column that references the primary key on the department_id column of the departments table:

The constraint fk_deptno ensures that all departments given for employees in the dept_20 table are present in the departments table. However, employees can have null department numbers, meaning they are not assigned to any department. To ensure that all employees are assigned to a department, you could create a NOT NULL constraint on the department_id column in the dept_20 table in addition to the REFERENCES constraint.

Before you define and enable this constraint, you must define and enable a constraint that designates the department_id column of the departments table as a primary or unique key.

The foreign key constraint definition does not use the FOREIGN KEY clause, because the constraint is defined inline. The data type of the department_id column is not needed, because Oracle automatically assigns to this column the data type of the referenced key.

The constraint definition identifies both the parent table and the columns of the referenced key. Because the referenced key is the primary key of the parent table, the referenced key column names are optional.

Alternatively, you can define this foreign key constraint out of line:

The foreign key definitions in both variations of this statement omit the ON DELETE clause, causing Oracle to prevent the deletion of a department if any employee works in that department.

ON DELETE Example This statement creates the dept_20 table, defines and enables two referential integrity constraints, and uses the ON DELETE clause:

Because of the first ON DELETE clause, if manager number 2332 is deleted from the employees table, then Oracle sets to null the value of manager_id for all employees in the dept_20 table who previously had manager 2332.

Because of the second ON DELETE clause, Oracle cascades any deletion of a department_id value in the departments table to the department_id values of its dependent rows of the dept_20 table. For example, if Department 20 is deleted from the departments table, then Oracle deletes all of the employees in Department 20 from the dept_20 table.

Composite Foreign Key Constraint Example The following statement defines and enables a foreign key on the combination of the employee_id and hire_date columns of the dept_20 table:

The constraint fk_empid_hiredate ensures that all the employees in the dept_20 table have employee_id and hire_date combinations that exist in the employees table. Before you define and enable this constraint, you must define and enable a constraint that designates the combination of the employee_id and hire_date columns of the employees table as a primary or unique key.

The EXCEPTIONS INTO clause causes Oracle to write information to the wrong_emp table about any rows in the dept_20 table that violate the constraint. If the wrong_emp exceptions table does not already exist, then this statement will fail.

Check Constraint Examples The following statement creates a divisions table and defines a check constraint in each column of the table:

Each constraint restricts the values of the column in which it is defined:

check_divno ensures that no division numbers are less than 10 or greater than 99.

check_divname ensures that all division names are in uppercase.

check_office restricts office locations to Dallas, Boston, Paris, or Tokyo.

Because each CONSTRAINT clause contains the DISABLE clause, Oracle only defines the constraints and does not enable them.

The following statement creates the dept_20 table, defining out of line and implicitly enabling a check constraint:

This constraint uses an inequality condition to limit an employee’s total commission, the product of salary and commission_pct , to $5000:

If an employee has non-null values for both salary and commission, then the product of these values must not exceed $5000 to satisfy the constraint.

If an employee has a null salary or commission, then the result of the condition is unknown and the employee automatically satisfies the constraint.

Because the constraint clause in this example does not supply a constraint name, Oracle generates a name for the constraint.

The following statement defines and enables a primary key constraint, two foreign key constraints, a NOT NULL constraint, and two check constraints:

The constraints enable the following rules on table data:

pk_od identifies the combination of the order_id and part_no columns as the primary key of the table. To satisfy this constraint, no two rows in the table can contain the same combination of values in the order_id and the part_no columns, and no row in the table can have a null in either the order_id or the part_no column.

fk_oid identifies the order_id column as a foreign key that references the order_id column in the orders table in the sample schema oe . All new values added to the column order_detail . order_id must already appear in the column oe.orders.order_id .

fk_pno identifies the product_id column as a foreign key that references the product_id column in the product_information table owned by oe . All new values added to the column order_detail.product_id must already appear in the column oe.product_information.product_id .

nn_qty forbids nulls in the quantity column.

check_qty ensures that values in the quantity column are always greater than zero.

check_cost ensures the values in the cost column are always greater than zero.

This example also illustrates the following points about constraint clauses and column definitions:

Out-of-line constraint definition can appear before or after the column definitions. In this example, the out-of-line definition of the pk_od constraint precedes the column definitions.

A column definition can contain multiple inline constraint definitions. In this example, the definition of the quantity column contains the definitions of both the nn_qty and check_qty constraints.

A table can have multiple CHECK constraints. Multiple CHECK constraints, each with a simple condition enforcing a single business rule, are preferable to a single CHECK constraint with a complicated condition enforcing multiple business rules. When a constraint is violated, Oracle returns an error identifying the constraint. Such an error more precisely identifies the violated business rule if the identified constraint enables a single business rule.

Attribute-Level Constraints Example The following example guarantees that a value exists for both the first_name and last_name attributes of the name column in the students table:

REF Constraint Examples The following example creates a duplicate of the sample schema object type cust_address_typ , and then creates a table containing a REF column with a SCOPE constraint:

The following example creates the same table but with a referential integrity constraint on the REF column that references the object identifier column of the parent table:

The following example uses the type department_typ and the table departments_obj_t , created in «Creating Object Tables: Examples». A table with a scoped REF is then created.

The following statement creates a table with a REF column which has a referential integrity constraint defined on it:

Explicit Index Control Example The following statement shows another way to create a unique (or primary key) constraint that gives you explicit control over the index (or indexes) Oracle uses to enforce the constraint:

This example also shows that you can create an index for one constraint and use that index to create and enable another constraint in the same statement.

DEFERRABLE Constraint Examples The following statement creates table games with a NOT DEFERRABLE INITIALLY IMMEDIATE constraint check (by default) on the scores column:

To define a unique constraint on a column as INITIALLY DEFERRED DEFERRABLE , issue the following statement:

Do You Have REFERENCES?

The logic for referential integrity can be implemented in application code, but to make sure that it is enforced, include foreign key constraints in the database design instead. In this article, Joe Celko talks about the history of references in SQL and the options available today.

The late Jim Gray once said that in the early days of SQL, “We had no idea what we were doing!” However, that is not completely true. What we were doing was mimicking the technologies that had gone before. The first SQL engines put each table in a separate physical file. We had file systems that had been in use for decades. We had lots of code for handling those files, in particular, all kinds of variations on index sequential access methods (ISAM). But data modeling introduced something we hadn’t had before: the concept of data integrity being enforced declaratively instead of procedurally.

In the dark ages of file systems, if we wanted to restrict a field in a record to particular values, then we had to have a program to enforce this rule. Actually, it was worse than that because we had to have every program enforce this rule if it made a modification in the file. The idea of having a general CHECK() constraint on a column simply did not exist. COBOL gave us some display formatting on fields with the PICTURE clause, but this had nothing to do with the relationships in the data.

Here’s a relatively straightforward example from the old days. You have an inventory file that shows all the goods that you sell and an orders file that shows who placed what orders. The integrity rule is pretty simple: you can’t sell anything that you don’t have in the inventory. You would go to the Orders file record, loop through the items that were ordered, which would be in a repeating group called the OCCURS clause in COBOL and match them to the inventory. If you had the item in inventory, you would execute one procedure (in COBOL, this would be a PERFORM paragraph statement). If you didn’t have items, you would execute a second procedure.

REFERENCES Clause

The <references specification> is the simplest version of a referential constraint definition:

What this says is that the value in this column of the referencing table must appear somewhere in the referenced table’s columns which are named in the constraint. Notice the terms “referencing” and “referenced” are not the same as the “parent” and “child” terms used in network databases. Those terms were based on pointer chains that were traversed in one direction; that is, you cannot find a path back to the parent node from a child node in the network. Another difference is that the referencing and referenced tables can be the same table. There is also no such thing as a “link table” in RDBMS; that’s another network database term.

Furthermore, the referenced column must have a UNIQUE constraint. A PRIMARY KEY is a special case of a UNIQUE constraint that also implied NOT NULL on all its columns. If the referenced columns are in a UNIQUE constraint, then the target table must have one and only one NULL in that column. The NULLs will match in the referencing table. If no <reference column list> is given, then the PRIMARY KEY of the referenced table is assumed to be the target. There is no rule to prevent several columns from referencing the same target columns. For example, you might have a table of flight crews that has pilot and copilot columns that both reference a table of certified pilots. A table can also reference itself (this can get tricky and involves turning constraints on and off). A circular reference is a relationship in which one table references a second table, which in turn references the first table. The old gag about “you cannot get a job until you have experience, and you cannot get experience until you have a job!” is the classic version of this.

As a general design principle, it’s much more convenient to have a tree structured span of references. In particular, it makes referential actions much more predictable. Now I need to define “referential actions” and show how they work.

Referential Actions

The very first SQL engines behaved pretty much like procedural code language files. When TRIGGERs were added to the language, you could still do integrity checks in procedural code, but now it was in one place, the DDL, and not have to repeat it in every module of code. But people began to notice the same coding patterns were being used over and over in about 80% of these TRIGGERs. So, we added declarative subclauses for the most common situations. This means that the SQL engine can optimize these cases, which is not possible with triggers.

We decided that the REFERENCES clause can have two sub-clauses that take actions when a “database event” changes the referenced table. The two database events are updates and deletes and the sub-clauses look like this:

When the referenced table is changed, one of the referential actions is set in motion by the SQL engine.

1) The CASCADE option will change the values in the referencing table to match the value (if any) in the referenced table. This is a very common programming technique that allows you to set up a single table as the trusted source for an identifier. This way, the system can propagate changes automatically.

The ON DELETE CASCADE is probably the most common option. The reason is that in data modeling we talk about having “strong” and “weak” entities. A weak entity (such as the Order Details) can exist only if they have a reference back to a strong entity (Orders). You can build chains of weaker and weaker entity references to any depth and spread it out in a tree structure that begins at the strongest entity. Let’s use ← to mean “references” and look at the possible ways you can chain a strong entity, E1, and it’s two weaker entities, E2 and E3.

The difference can be subtle. Imagine that E1 is an order. In the first case, E2 might be order items like a back-to-school supply kit. This kit is made up of individual items (pencils, pens, crayons, paper, etc.) from E3. In this model, you can delete from or add individual items to a kit. Whatever you do, it’s still a back-to-school kit until you remove all the items.

In the second case, E2 might be an order item, and E3 could be delivery options. In theory, you could have an order, E1, that is empty and still deliver it. That doesn’t make much sense in the real world, but it is allowed by the data model.

2) The SET NULL option will change the values in the referencing table to a NULL . Obviously, the referencing column needs to be NULL-able, but the referenced column does not.

3) The SET DEFAULT option will change the values in the referencing table to the default value of that column. Obviously, the referencing column needs to have some DEFAULT declared for it, but each referencing column can have its own default in its own table.

A little-known feature of SQL is the DEFAULT VALUES clause in the INSERT INTO statement where a single row is inserted containing only DEFAULT values for every column. The syntax is: INSERT INTO <table name> DEFAULT VALUES ; as a shorthand for INSERT INTO <table name> VALUES (DEFAULT, DEFAULT,… DEFAULT) .

4) The NO ACTION option explains itself. Nothing is changed in the referencing table, and a warning message about reference violation might be raised. If a REFERENCES constraint does not specify any ON UPDATE or ON DELETE subclause, then NO ACTION is implicit.

Full ANSI/ISO Standard SQL has more options about how matching is done between the referenced and referencing tables. Full ANSI/ISO Standard SQL also has deferrable constraints. This lets the programmer turn a constraint off during a session so that the table can be put into a state that would otherwise be illegal. However, at the end of a session, all the constraints are enforced. Many SQL products have implemented these options, and they can be quite handy, but I will not mention them anymore. In SQL Server, you have to explicitly turn the constraints on and off with the statement. Please remember to give your constraints names, so this feature will be easy to use.

Что означает CONSTRAINT и REFERENCES в данном запросе?

CONSTRAINT — ключевое слово, которое указывает, что в данной секции описывается ограничение, которое налагается на данные в таблице, и которое будет проверяться подсистемой контроля целостности и непротиворечивости данных сервера, что в свою очередь не позволит внести или изменить данные в таблице так, чтобы это условие не выполнялось, а при выявлении факта наличия не соответствующих этому условию данных в таблице она будет считаться повреждённой. Сразу после этого ключевого слова указывается уникальный идентификатор (имя) этого ограничения (он может быть пропущен, тогда сервер сгенерирует его автоматически).

REFERENCES — ключевое слово, которое указывает, что для каждой записи таблицы значение выражения описанного непосредственно перед ним индекса будет контролироваться на наличие в точности совпадающего по значению, включая деление на фрагменты при наличии такового, значения в уникальном индексном выражении таблицы, указанном непосредственно после этого ключевого слова. Контроль выполняется подсистемой контроля целостности и непротиворечивости данных сервера. Любые действия, которые приведут к нарушению такого соответствия, будут блокироваться. При выявлении любого несоответствия данные будут считаться повреждёнными.

Это ограничение, в данном случае внешний ключ. То есть значение поля TeamId ограничивается значениями содержащимися в поле Id , таблицы Teams , либо значением NULL .

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References sql что это