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Editorial Team
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Asked: 2026-05-27T11:04:05+00:00

I had a strange error where I did something like the following: SELECT *

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I had a strange error where I did something like the following:

SELECT *
FROM table1
WHERE NAME COLLATE LATIN1_General_CI_AS NOT IN (select name FROM table2)

Name was a nvarchar(50) null. Table2 had null values in the name column. It would then match names that did not exist in table2 because of the null values. Meaning that this would return 0 results. And if I did:

SELECT *
FROM table1
WHERE NAME COLLATE LATIN1_General_CI_AS NOT IN (
  select name FROM table2 where name is not null
)

I would get some results.

One table had collation Danish_Norwegian_CI_AS and the other one latin1. Maybe the COLLATE is the source of the problem?

Can anyone explain why this happens? Maybe something with set theory?

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1 Answer

  1. Editorial Team

    It has nothing to do with collation. Try set ansi_nulls off and the first query will work as you expected.

    The reason is that when you try to compare something to NULL (e.g. a = NULL or a <> NULL) the result is UNKNOWN and the entire query fails. If you set ansi_nulls to off then the same comparison evaluates to TRUE or FALSE and the query “works”.

    In other words:

    WHEN a NOT IN (1, 2, null)

    means

    WHEN a <> 1 AND a <> 2 AND a <> null

    WHERE the last part a <> null evaluates to UNKNOWN and the query fails to return any rows.

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