In TSQL, will the following examples yield different results? - tsql

Question

I have to change an update query in TSQL to include the following code:
(uh.FirstName != cu.FirstName or isnull(uh.FirstName,'|||') != isnull(cu.Firstname,'|||')
But to my mind that seems redundant and it would be better to just use the second part:
isnull(uh.FirstName,'|||') != isnull(cu.Firstname,'|||')
Am I missing something?

Answer 1

It looks like the ISNULL(x,'|||') is just trying to handle considering nulls equivalent. You have to handle each null case and potentially double null depending on whether that is a "match" or not. Here's my suggestion with the commented line being option depending on how you want to handle double null.
(
uh.FirstName != cu.FirstName
OR uh.FirstName IS NULL AND cu.FirstName IS NOT NULL
OR uh.FirstName IS NOT NULL AND cur.FirstName IS NULL
--OR uh.FirstName IS NULL AND cur.FirstName IS NULL
)
. Not sure whether the coalesce is faster than substitution and comparison but my guess is it would be. It also doesn't allow for ambiguity comparing '|||' to NULL which would yield false results with that solution.
*Fixed after HABO's comment

Answer 2

Unless you expect any of the columns to actually contains the literal string value of three pipes ('|||'), you can use the second code snippet safely.
Quick demo:
DECLARE #T AS TABLE
(
c1 varchar(5),
c2 varchar(5)
);
INSERT INTO #T VALUES
('a', 'a'),
('a', 'b'),
('a', null),
(null, 'a'),
(null, null),
('b', 'a');
SELECT *, 'First' as example
FROM #T
WHERE c1 != c2 OR ISNULL(c1, '|||') != ISNULL(c2, '|||')
SELECT *, 'Second' as example
FROM #T
WHERE ISNULL(c1, '|||') != ISNULL(c2, '|||')
Results:
c1 c2 example
a b First
a NULL First
NULL a First
b a First
c1 c2 example
a b Second
a NULL Second
NULL a Second
b a Second