I understand that SQL EXISTS checks for the existence of rows, however does it evaluate the entire expression? So for example, would something like this:
IF EXISTS (SELECT TOP 1 1 FROM table WITH (NOLOCK)) BEGIN ... END Be faster than something like this:
IF EXISTS (SELECT 1 FROM table WITH (NOLOCK)) BEGIN ... END Exists will stop after the first hit because then the expression evaluates to true, so the top(1)-part is unnecessary.
Both those should run exactly the same. SQL Server takes into account that EXISTS is a short-circuited operation and doesn't evaluate the return result, just checks to see if there IS a return result!
No, it won't.
SQL Server uses TOP in the plan to evaluate EXISTS.
The statements generate identical query plans so there is no difference. The second example is easier to read in my opinion.
