Nulls and the MERGE statement: I need to set a value to infinity. How?

Question

In SQL Server 2008, I\'m using MERGE. Everything is fine except that I have 2 nullable columns. If I pass a null value and the target isn\'t null, MERGE doesn\'t see a diffe

Answer 1

You can change the ON part of the merge statement, putting in a check for when both source and target are null.

MERGE tgt
USING src
ON ( -- enter non-nullable columns to match on ...
    tgt.A = src.A
    AND (tgt.C = src.C OR (tgt.C IS NULL AND src.C IS NULL))
)
WHEN MATCHED -- ...

Answer 2

Instead of using 0 when the values are null, why not use a value that is highly unlikely to exist? EG (IsNull(tgt.C, 2093128301).

The datatypes are int so you have a lot to play with......

Answer 3

WHEN MATCHED AND tgt.c <> src.c OR tgt.c IS NULL AND src.c IS NOT NULL OR tgt.c IS NOT NULL AND src.c IS NULL

Answer 4

This works as well and may be better when you have multiple columns that you want to check if they are different.

  MERGE @t2 a

  using @t1 b

  ON a.PK = b.PK

  WHEN MATCHED AND CHECKSUM(a.PK,a.VALUE)!= CHECKSUM(b.pk,b.VALUE)

  THEN UPDATE SET a.VALUE = b.VALUE;

Answer 5

Have you tried SET ANSI_NULLS OFF, which will make NULL=NULL return true? This may create additional issues but it could be a script-level workaround (turn it off then on once you run your proc).

Answer 6

You can check for null in the ON Clause:

MERGE TargetTable
USING (VALUES (0)) as s(x)
ON last_run is not null
WHEN not matched then
insert (last_run) values(getdate())
when matched then
update set last_run=getDate();