Performance in a case statement

Question

Hi
I have a query like this -
SELECT
MAX(CASE WHEN field1 = 'xyz' THEN field2 END) AS alias1,
MAX(CASE WHEN field2 = 'abc' THEN field2 END) AS alias2
FROM table1
WHERE
field1 = 'xyz' and
field2 = 'abc' and
some fixed condition
Please note that the conditions in where clauses are same as corresponding case statements apart from one fixed condition.
In the actual query there are ~15 such conditions and 15 case statements for each selected field.
I have put the where clause to reduce the number of rows getting returned from the table.
My question is - what happens in terms of performance if I remove the where clause here. In other words will above query perform better or same as this
SELECT
MAX(CASE WHEN field1 = 'xyz' THEN field2 END) AS alias1,
MAX(CASE WHEN field2 = 'abc' THEN field2 END) AS alias2
FROM table1
WHERE
some fixed condition
Thanks
Kumar

Answer 1

It really depends on what you query is trying to accomplish. The two queries are not the same. Your first will select rows where both field1 = 'xyz' and field2 = 'abc', while the second would also return rows where, for example, field1 = 'xyz' and field2 = 'def'. In which case, your alias2 would be null.
John