How to compare values which may both be null in T-SQL

Question

I want to make sure I\'m not inserting a duplicate row into my table (e.g. only primary key different). All my fields allow NULLS as I\'ve decided null to mean \"all values

Answer 1

You create a primary key on your fields and let the engine enforce the uniqueness. Doing IF EXISTS logic is incorrect anyway as is flawed with race conditions.

Answer 2

You could use SET ANSI_NULLS in order to specify the behavior of the Equals (=) and Not Equal To (<>) comparison operators when they are used with null values.

Answer 3

Did you check NULLIF? http://msdn.microsoft.com/en-us/library/ms177562.aspx

Answer 4

NULLIF(TARGET.relation_id, SOURCE.app_relation_id) IS NULL Simple solution

Answer 5

You will have to use IS NULL or ISNULL. There really isn't a away around it.

Answer 6

I needed a similar comparison when doing a MERGE:

WHEN MATCHED AND (Target.Field1 <> Source.Field1 OR ...)

The additional checks are to avoid updating rows where all the columns are already the same. For my purposes I wanted NULL <> anyValue to be True, and NULL <> NULL to be False.

The solution evolved as follows:

First attempt:

WHEN MATCHED AND
(
    (
        -- Neither is null, values are not equal
        Target.Field1 IS NOT NULL
            AND Source.Field1 IS NOT NULL
            AND Target.Field1 <> Source.Field1
    )
    OR
    (
        -- Target is null but source is not
        Target.Field1 IS NULL
            AND Source.Field1 IS NOT NULL
    )
    OR
    (
        -- Source is null but target is not
        Target.Field1 IS NOT NULL
            AND Source.Field1 IS NULL
    )

    -- OR ... Repeat for other columns
)

Second attempt:

WHEN MATCHED AND
(
    -- Neither is null, values are not equal
    NOT (Target.Field1 IS NULL OR Source.Field1 IS NULL)
        AND Target.Field1 <> Source.Field1

    -- Source xor target is null
    OR (Target.Field1 IS NULL OR Source.Field1 IS NULL)
        AND NOT (Target.Field1 IS NULL AND Source.Field1 IS NULL)

    -- OR ... Repeat for other columns
)

Third attempt (inspired by @THEn's answer):

WHEN MATCHED AND
(

    ISNULL(
        NULLIF(Target.Field1, Source.Field1),
        NULLIF(Source.Field1, Target.Field1)
    ) IS NOT NULL

    -- OR ... Repeat for other columns
)

The same ISNULL/NULLIF logic can be used to test equality and inequality:

• Equality: ISNULL(NULLIF(A, B), NULLIF(B, A)) IS NULL
• Inequaltiy: ISNULL(NULLIF(A, B), NULLIF(B, A)) IS NOT NULL

Here is an SQL-Fiddle demonstrating how it works http://sqlfiddle.com/#!3/471d60/1