What is the best way to equate NULL columns in SQL? - sql

Question

I am performing a MERGE between two tables
MERGE indexdecomp.Constituent targ
USING (SELECT ic.ConstituentName
FROM indexdecomp.IndexConstituents ic) src
ON (((targ.Name = src.ConstituentName) OR (targ.Name IS NULL AND src.ConstituentName IS NULL)))
WHEN NOT MATCHED BY TARGET THEN
UPDATE SET
targ.Name = src.ConstituentName
;
and in my ON clause I have the following predicate:
(targ.Name = src.ConstituentName) OR (targ.Name IS NULL AND src.ConstituentName IS NULL)
I have this predicate since I am considering it a match if both Names are equal or if both names are `null.
Is there a better or more conventional way to handle equality between two null columns? What way would produce the quickest execution?

Answer 1

You can do something like this: (SQL ref)
SET ANSI_NULLS OFF;
MERGE indexdecomp.Constituent targ
USING (SELECT ic.ConstituentName
FROM #IndexConstituents ic) src
ON (((targ.Name = src.ConstituentName)))
WHEN NOT MATCHED BY TARGET THEN
UPDATE SET
targ.Name = src.ConstituentName;
SET ANSI_NULLS ON;
But that seems to be a pretty heavy trade off for lumping off a predicate and neither is very readable. You could actually abstract this mess with a UDF that takes two string arguments and returns a boolean.
Something like:
create function StrNullCompare(#a varchar(max), #b varchar(max))
returns int
as
begin
if ((#a = #b) or (#a is null and #b is null)) return 1;
return 0;
end
-- tests
select dbo.StrNullCompare('wer', 'were');
select dbo.StrNullCompare('wer', 'wer');
select dbo.StrNullCompare('hi', null);
select dbo.StrNullCompare(null, null);
And your predicate becomes:
(dbo.StrNullCompare(targ.Name, src.ConstituentName)=1)

Answer 2

You could try..
ISNULL (targ. Name,'a magic string value') =ISNULL (src.ConstituentName,'a magic string value')
Of course add your own magic string as appropriate for example use newid () to get a guid and use that.
Not really sure if this is "better" than an and or but is a little more human readable; worth benchmarking and testing both approaches though.