NOT IN clause and NULL values
https://stackoverflow.com/questions/129077/not-in-clause-and-null-values
This issue came up when I got different records counts for what I thought were identical queries one using a not in
where
constraint and the other a left join
. The table in the not in
constraint had one null value (bad data) which caused that query to return a count of 0 records. I sort of understand why but I could use some help fully grasping the concept.
To state it simply, why does query A return a result but B doesn't?
A: select 'true' where 3 in (1, 2, 3, null)
B: select 'true' where 3 not in (1, 2, null)
This was on SQL Server 2005. I also found that calling set ansi_nulls off
causes B to return a result.
答案:
Query A is the same as:
select 'true' where 3 = 1 or 3 = 2 or 3 = 3 or 3 = null
Since 3 = 3
is true, you get a result.
Query B is the same as:
select 'true' where 3 <> 1 and 3 <> 2 and 3 <> null
When ansi_nulls
is on, 3 <> null
is UNKNOWN, so the predicate evaluates to UNKNOWN, and you don't get any rows.
When ansi_nulls
is off, 3 <> null
is true, so the predicate evaluates to true, and you get a row.