07-31-2023, 09:46 AM
Whenever you use NULL you are really dealing with a Three-Valued logic.
Your first query returns results as the WHERE clause evaluates to:
3 = 1 or 3 = 2 or 3 = 3 or 3 = null
which is:
FALSE or FALSE or TRUE or UNKNOWN
which evaluates to
TRUE
The second one:
3 <> 1 and 3 <> 2 and 3 <> null
which evaluates to:
TRUE and TRUE and UNKNOWN
which evaluates to:
UNKNOWN
The UNKNOWN is not the same as FALSE
you can easily test it by calling:
select 'true' where 3 <> null
select 'true' where not (3 <> null)
Both queries will give you no results
If the UNKNOWN was the same as FALSE then assuming that the first query would give you FALSE the second would have to evaluate to TRUE as it would have been the same as NOT(FALSE).
That is not the case.
There is a very good [article on this subject on SqlServerCentral][1].
The whole issue of NULLs and Three-Valued Logic can be a bit confusing at first but it is essential to understand in order to write correct queries in TSQL
Another article I would recommend is [SQL Aggregate Functions and NULL][2].
[1]:
Your first query returns results as the WHERE clause evaluates to:
3 = 1 or 3 = 2 or 3 = 3 or 3 = null
which is:
FALSE or FALSE or TRUE or UNKNOWN
which evaluates to
TRUE
The second one:
3 <> 1 and 3 <> 2 and 3 <> null
which evaluates to:
TRUE and TRUE and UNKNOWN
which evaluates to:
UNKNOWN
The UNKNOWN is not the same as FALSE
you can easily test it by calling:
select 'true' where 3 <> null
select 'true' where not (3 <> null)
Both queries will give you no results
If the UNKNOWN was the same as FALSE then assuming that the first query would give you FALSE the second would have to evaluate to TRUE as it would have been the same as NOT(FALSE).
That is not the case.
There is a very good [article on this subject on SqlServerCentral][1].
The whole issue of NULLs and Three-Valued Logic can be a bit confusing at first but it is essential to understand in order to write correct queries in TSQL
Another article I would recommend is [SQL Aggregate Functions and NULL][2].
[1]:
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[2]:[To see links please register here]