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
[Edit] Roji. P. Thomas has an excellent article with even more detailed examples. The links is below.