Check if a record exist return an incorrect result

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I have a table called match_lineup_substitute which have the following record:

player_substitute_id | player_substituted_id | match_id |
       90930                   NULL             2807486

I want check if the record exist, so I wrote this query:

SELECT COUNT(*) FROM `match_lineup_substitute` 
                WHERE match_id = 2807486 AND 
                player_substitute_id = 90930 AND player_substituted_id = null         

this will return: 0

what happened?

Change from

player_substituted_id = null 

to

player_substituted_id IS NULL 

If you want to check null value in field you have to use IS NULL in where clause.

How to find out if a record already exists in a database, If it doesn't , How do you check if record already exists in SQL? How can I check to see if a Record exists for this particular response, if not then create a new record. I was thinking of testing by using the lookup function, to see if a record existed that had the Question ID and User().Email, as the combination of these would be unique for each record.

To check for null values use is

... and player_substituted_id IS NULL

The DOC

Best way to test if a row exists in a MySQL table, How do you check if a row exists in SQL? If you are only checking that the record exists then you can try to use select 1 instead of select * Also the syntax is presently wrong, it should be select * from table1(you are missing the from) – Rahul Tripathi Jan 29 '16 at 11:45

NULL is not a value (see it as "value is unknown"), therefore column = NULL always returns NULL.

NULL AND something returns NULL

So your WHERE condition doesn't return TRUE (since it returns NULL), so the row is not selected.

Use column IS NULL instead.

Note that the same is true if you compare two columns : column1 = column2will be NULL for each row where column1 or column2 is NULL.

see https://dev.mysql.com/doc/refman/5.7/en/working-with-null.html

Return Boolean Value on SQL Select Statement, The value will be returned as 1 (True) if record exists and 0 (False) is record of assigning result value EXEC function to Variable in SQL Server, the following  DECLARE @CustId INT = 2 IF EXISTS(SELECT 1 FROM dbo.Customer WITH(NOLOCK) WHERE CustId = @CustId) BEGIN PRINT 'Record Exists' END ELSE BEGIN PRINT 'Record doesn''t Exists' END RESULT: [ALSO READ] How to check if a Table exists

In MySQL you should use IS NULL (or IS NOT NULL) to evaluate if a value is (or isn't) null, instead of using arithmetic comparison operators.

In your example, you should replace

AND player_substituted_id = null

with:

... AND player_substituted_id IS NULL 

You can read more about it in the Working with NULL values section of the official documentation.

...

To test for NULL, use the IS NULL and IS NOT NULL operators (...)

...

You cannot use arithmetic comparison operators such as =, <, or <> to test for NULL.

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Comments
  • To check for null values, we use IS NULL not = null
  • I'd avoid such similar column names. player_substitute_id and player_substituted_id will cause trouble.
  • Possible duplicate of: stackoverflow.com/questions/2749044/what-is-null-and-is-null
  • NULL = NULL yields NULL which isn't TRUE. NULL IS NULL yields TRUE , which is useful to you. Most operations on NULL yield NULL and need very specific care in handling them.