Carlo Romagnano (11/3/2009)
Noel McKinney (11/3/2009)
This is the main reason I make it a habit to use COALESCE rather than ISNULL unless there's a reason I need the behavior of ISNULL.
I prefer ISNULL because of performance.
So, do you have any repro code that actually demonstrates a performance difference between COALESCE and ISNULL? If so, please post!
The only situation I know of is when an expression in COALESCE is a subquery - in those cases, ISNULL can be found to perform better. But how often does one actuallly need an entire subquery in COALESCE or ISNULL?
Like Noel, I use COALESCE rather than ISNULL. Because of the weird datatyping rules of ISNULL, but also because it supports more than two arguments, and because it's ANSI standard. The only reasons I'd ever prefer ISNULL are
1) If I have to use it in a view or computed column where the column has to be not nullable - any COALESCE expression is always nullable; ISNULL is only nullable is the second argument is nullable.
2) If any of the arguments has to be a subquery, because of the potential performance problems COALESCE has with those - and since this is acutally caused by an optimizer weakness, I'd make a note to check if the optimizer has finally improved in this respect after every version or servicepack upgrade.
Hugo Kornelis, SQL Server MVP
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