Monday, June 12, 2006

COALESCE And ISNULL Differences

I decided to do a quick post about two differences between COALESCE and ISNULL

Run the following block of code

-- The result is 7, integer math
SELECT 15 / ISNULL(CONVERT(INT,NULL), 2.00)

--The result is 7.5, which is correct
SELECT 15 / COALESCE(CONVERT(INT,NULL), 2.00)

You will see that the result is not the same ISNULL does integer math while COALESCE does not

COALESCE correctly promotes its arguments to the highest data type in the expression list.
ISNULL just looks at the first datatype, which is an integer (15) and makes everything an int
COALESCE looks at 2.00 and 15 and then promotes the integer to decimal

Another example is returning the first non null value, ISNULL can only take 2 values while COALESCE can take a whole lot more
Here we have 4 variables and all except for one are null

DECLARE @Var1 VARCHAR(20)
DECLARE @Var2 VARCHAR(20)
DECLARE @Var3 VARCHAR(20)
DECLARE @Var4 VARCHAR(20)

SELECT @Var4 = 'ABC'

--This will return ABC
SELECT COALESCE(@Var1,@Var2,@Var3,@Var4)

 Roji. P. Thomas has an excellent article with even more detailed examples. The links is below.
http://toponewithties.blogspot.com/2004/08/differences-between-coalesce-and.html
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Anonymous said...

Someday remembering what you have mentioned here would probably save a person hours of wondering why the heck this code isn't exactly right.

Too bad the panic will have set in by then removing all logic as to why it might be occurring :)

Denis said...

Yep, this is just another reason to stay away from proprietary syntax. Although I must admit I do use GETDATE() instead of CURRENT_TIMESTAMP() in my code

Anonymous said...

I thought it was just CURRENT_TIMESTAMP without the () parentheses...

Denis said...

True should be CURRENT_TIMESTAMP not CURRENT_TIMESTAMP()

You can tell I use GETDATE()

Roji. P. Thomas said...

http://toponewithties.blogspot.com/2004/08/differences-between-coalesce-and.html

Denis said...