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Difference between two expression results with a NULL value

Steilla Mitchel 312 30-Jul-2023

The difference between two expression results with a NULL value

mssql server sql server  sql 
Updated on 31-Jul-2023
Steilla Mitchel

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Aryan Kumar

Aryan Kumar

31-Jul-2023

Sure, here is the difference between two expression results with a null value in SQL:

  • == will return NULL if either of the expressions is null. This is because the == operator only returns TRUE if both of the expressions are equal. If one of the expressions is null, then the == operator will return NULL.
  • IS NULL will return TRUE if the expression is null and FALSE if the expression is not null. This is because the IS NULL operator specifically checks if the expression is null.
  • !== will return TRUE if either of the expressions is not null and FALSE if both of the expressions are null. This is because the !== operator only returns TRUE if the expressions are not equal. If one of the expressions is null, then the !== operator will return FALSE.

Here is an example of how the three operators would work:

SQL

SELECT
  expression1,
  expression2,
  expression1 = expression2,
  expression1 IS NULL,
  expression2 IS NULL;

If expression1 and expression2 are both null, then the following results will be returned:

expression1 | expression2 | expression1 = expression2 | expression1 IS NULL | expression2 IS NULL
---------- | -------- | ------------------- | --------------- | ---------------
NULL       | NULL       | NULL                | TRUE             | TRUE

If expression1 is null and expression2 is not null, then the following results will be returned:

expression1 | expression2 | expression1 = expression2 | expression1 IS NULL | expression2 IS NULL
---------- | -------- | ------------------- | --------------- | ---------------
NULL       | 'hello'   | NULL                | TRUE             | FALSE

If expression1 is not null and expression2 is null, then the following results will be returned:

expression1 | expression2 | expression1 = expression2 | expression1 IS NULL | expression2 IS NULL
---------- | -------- | ------------------- | --------------- | ---------------
'hello'   | NULL       | NULL                | FALSE            | TRUE
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