function use in the SELECT List and or on the JOIN can also cause severe problems (maybe even worse then in the WHERE Clause).

by placing a Function in the select, that function will execute 1x per row.

We had these at my work also when I first got there. They were causing terrible performance system wide.

Removing them all helped immensely. typically you can replace the functions with some other mechanism of retrieving the data needed (Derived tables, correlated subQueries, etc)

you really need to pay close attention to the Query Plan and specifically to the SubTree cost of the plan. If the SubTree cost approaches 10.0 (in my experience), you need to consider modifying the query.

Cheers,

GAJ