NULL-значения в SQL
Достаточно часто встречаются такие случаи, когда в таблице имеются записи с не заданными значениями какого-либо из полей, потому что значение поля неизвестно или его просто нет. В таких случаях SQL позволяет указать в поле NULL-значение. Строго говоря, NULL-значение вовсе не представлено в поле. Когда значение поля есть NULL — это значит, что программа базы данных специальным образом помечает поле, как не содержащее какого-либо значения для данной строки (записи).
Дело обстоит не так в случае простого приписывания полю значения «нуль» или «пробел», которые база данных трактует как любое другое значение. Поскольку NULL не является значением как таковым, он не имеет типа данных. NULL может размещаться в поле любого типа. Тем не менее, NULL, как NULL-значение, часто используется в SQL.
Предположим, появился покупатель, которому еще не назначен продавец. Чтобы констатировать этот факт, нужно ввести значение NULL в поле snum, а реальное значение включить туда позже, когда данному покупателю будет назначен продавец.
SQL IS NULL
Поскольку NULL фиксирует пропущенные значения, результат любого сравнения при наличии NULL-значений неизвестен. Когда NULL-значение сравнивается с любым значением, даже с NULL-значением, результат просто неизвестен. Булево значение «неизвестно» ведет себя также, как «ложь» — строка, на которой предикат принимает значение «неизвестно», не включается в результат запроса – при одном важном исключении: NOT от лжи есть истина (NOT (false)=true), тогда как NOT от неизвестного значения есть также неизвестное значение. Следовательно, такое выражение как «city = NULL» или «city IN (NULL)» является неизвестным независимо от значения city.
Часто необходимо различать false и unknown – строки, содержащие значения столбца, не удовлетворяющие предикату, и строки, которые содержат NULL. Для этой цели SQL располагает специальным оператором IS, который используется с ключевым словом NULL для локализации NULL-значения. SQL IS NULL. Пример. Вывести все поля из талицы Customers, значения поля city которых равны NULL:
SQL ignore part of WHERE if parameter is null
I have a stored procedure that fetches info from a table based on 4 parameters.
I want to get values based on the parameters, but if a parameter is NULL then that parameter isn’t checked. So if all 4 parameters is null I would show the entire table.
This is my SP (as you can see, this only works for 1 parameter atm):
Is there some way to do this without having a IF for every possible combination (15 IFs)?
8 Answers 8
How about something like
in this specific case you could have also used
But in general you can try something like
You can use use COALESCE() function in SQL server. You don’t need to use IF- Else or CASE in your statements. Here is how you can use COALESCE function.
The COALESCE function in SQL returns the first non-NULL expression among its arguments. Here for example if the @param1 is equal to null the function will return col1 which will lead to col1=col1 in the where statement which is like 1=1 meaning the condition will always be true.
How I Learned to Stop Worrying and Love NULL in SQL
You should not try to prevent NULL values — instead, write your query in a way to overcome its limitations.
2 years ago • 6 min read
The NULL value is a data type that represents an unknown value. It is not equivalent to empty string or zero. Suppose you have an employee table containing columns such as EmployeeId , Name , ContactNumber and an alternate contact number. This table has a few mandatory value columns like EmployeeId , Name , and ContactNumber . However, an alternate contact number is not required and therefore has an unknown value. Therefore a NULL value in this table represents missing or inadequate information. Here are other meanings NULL can have:
- Value Unknown
- Value not available
- Attribute not applicable
In this post we will consider how NULL is used in creating tables, querying, string operations, and functions. Screenshots in this post come from the Arctype SQL Client.
Allowing NULL in CREATE TABLE
To a table structure, we need to define whether the respective column allows NULL or not. For example, look at the following customer's table. The columns such as CustomerID , FirstName , LastName do not allow NULL values, whereas the Suffix , CompanyName , and SalesPerson columns can store NULL values.
Let’s insert a few records into this table using the following script.
Using NULL in the WHERE Clause
Now, suppose you want to fetch records for those customers who do not have an email address. The following query works fine, but it will not give us a row:
Values that are NULL cannot be queried using =
In the above select statement expression defines “Where the email address equals an UNKNOWN value”. In the SQL standard, we cannot compare a value to NULL. Instead, you refer to the value as IS NULL for this purpose. Note: There is a space between IS and NULL. If you remove space, it becomes a function ISNULL().
By using IS NULL instead of equals you can query for NULL values.
Integer, Decimal, and String Operations with NULL
Similarly, suppose you declared a variable but did not initialize its value. If you try to perform an arithmetic operation, it also returns NULL because SQL cannot determine the correct value for the variable, and it considers an UNKNOWN value.
Multiplying an integer by NULL returns NULL 
Multiplying a decimal by NULL returns NULL
NULL also plays an important role in string concatenation. Suppose you required the customer's full name in a single column, and you concatenate them using the pipe sign(||) .
Setting a string to NULL and then concatenating it returns NULL
Look at the result set — the query returns NULL in the concatenated string if any part of the string has NULL. For example, the person in Row 1 does not have a middle name. Its concatenated string is NULL as well, because SQL cannot validate the string value contains NULL.
There are many SQL functions available to overcome these NULL value issues in string concatenations. We’ll look at them later in this article.
The NULL value in SQL Aggregates
Suppose you use aggregate functions such as SUM, AVG, or MIN, MAX for NULL values. What do you think the expected outcome would be?
In aggregate functions NULL is ignored.
Look at the above figure: it calculated values for all aggregated functions. SQL ignores the NULLs in aggregate functions except for COUNT() and GROUP BY(). You get an error message if we try to use the aggregate function on all NULL values.
ORDER BY and GROUP BY with NULL
SQL considers the NULL values as the UNKNOWN values. Therefore, if we use ORDER By and GROUP by clause with NULL value columns, it treats them equally and sorts, group them. For example, in our customer table, we have NULLs in the MilddleName column. If we sort data using this column, it lists the NULL values at the end, as shown below.
NULL values appear last in ORDER BY
Before we use GROUP BY, let's insert one more record in the table. It has NULL values in most of the columns, as shown below.
Now, use the GROUP BY clause to group records based on their suffix.
GROUP BY does treat all NULL values equally.
As shown above, SQL treats these NULL values equally and groups them. You get two customer counts for records that do not have any suffix specified in the customers table.
Useful Functions for Working with NULL
We explored how SQL treats NULL values in different operations. In this section, we will explore a few valuable functions to avoid getting undesirable values due to NULL.
Using NULLIF in Postgres and MySQL
The NULLIF() function compares two input values.
● If both values are equal, it returns NULL.
● In case of mismatch, it returns the first value as an output.
For example, look at the output of the following NULLIF() functions.
NULLIF returns NULL if two values are equal 
NULLIF returns the first value if the values are not equal. 
NULLIF returns the first string in a string compare.
COALESCE function
The COALESCE() function accepts multiple input values and returns the first non-NULL value. We can specify the various data types in a single COALESCE() function and return the high precedence data type.
COALESCE returns the first non NULL data type in a list. 
Summary
The NULL value type is required in a relational database to represent an unknown or missing value. You need to use the appropriate SQL function to avoid getting undesired output for operations such as data concatenation, comparison, ORDER BY, or GROUP BY. You should not try to prevent NULL values — instead, write your query in a way to overcome its limitations. This way you will learn to love NULL.
Как в запросе sql исключить имена с null
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Question
May i know how to remove the null in column ? could i use case when condition ?
Answers
What you mean by removing null in column?
* If you don’t want any records with null value then
ex:select * from table where columnname is not null
* If you don’t want NULL value then use ISNULL function
ex:select col1,isnull(col2,»),isnull(col3,») from table
ESHANI. Please click "Mark As Answer" if a post solves your problem or "Vote As Helpful" if a post has been useful to you
- Marked as answer by Caulson Monday, September 30, 2013 5:22 AM
Are you looking for the below:
Please use Marked as Answer if my post solved your problem and use Vote As Helpful if a post was useful.
- Marked as answer by Caulson Monday, September 30, 2013 5:22 AM
All replies
you can write your query like:
Hope it gives you a basic idea.
Do not Forget to Vote as Answer/Helpful, please. It encourages us to help you.
What you mean by removing null in column?
* If you don’t want any records with null value then
ex:select * from table where columnname is not null
* If you don’t want NULL value then use ISNULL function
ex:select col1,isnull(col2,»),isnull(col3,») from table
ESHANI. Please click "Mark As Answer" if a post solves your problem or "Vote As Helpful" if a post has been useful to you
- Marked as answer by Caulson Monday, September 30, 2013 5:22 AM
1) If you want to remove NULL values from the table and wanting to replace with blank space, you can do that with update query
2) If you want to remove NULL values from the returned result set you can use ISNULL function or case statement if null value then ‘ ‘ in your selected statement
if the value is null then you can select blank space
Mark ANSWER if this reply resolves your query, If helpful then VOTE HELPFUL
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Are you looking for the below:
Please use Marked as Answer if my post solved your problem and use Vote As Helpful if a post was useful.
- Marked as answer by Caulson Monday, September 30, 2013 5:22 AM
Saeid Hasani, sqldevelop.wordpress.com
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