Which SQL statement is faster? (HAVING vs. WHERE...)

Question

SELECT NR_DZIALU, COUNT (NR_DZIALU) AS LICZ_PRAC_DZIALU
    FROM  PRACOWNICY
    GROUP BY NR_DZIALU
    HAVING NR_DZIALU = 30

or

SELECT NR_DZIALU, COUNT (NR_DZIALU) AS LICZ_PRAC_DZIALU
    FROM PRACOWNICY
    WHERE NR_DZIALU = 30
    GROUP BY NR_DZIALU

Answer 1

I'd expect the WHERE clause would be faster, but it's possible they'd optimize to exactly the same.

Answer 2

The theory (by theory I mean SQL Standard) says that WHERE restricts the result set before returning rows and HAVING restricts the result set after bringing all the rows. So WHERE is faster. On SQL Standard compliant DBMSs in this regard, only use HAVING where you cannot put the condition on a WHERE (like computed columns in some RDBMSs.)

You can just see the execution plan for both and check for yourself, nothing will beat that (measurement for your specific query in your specific environment with your data.)

Answer 3

The only way to find that out is to benchmark them.

Answer 4

It might depend on the engine. MySQL for example, applies HAVING almost last in the chain, meaning there is almost no room for optimization. From the manual:

The HAVING clause is applied nearly last, just before items are sent to the client, with no optimization. (LIMIT is applied after HAVING.)

I believe this behavior is the same in most SQL database engines, but I can't guarantee it.

Answer 5

The two queries are equivalent and your DBMS query optimiser should recognise this and produce produce the same query plan. It may not, but the situation is fairly simple to recognise, so I'd expect any modern system - even Sybase - to deal with it.

HAVING clauses should be used to apply conditions on group functions, otherwise they can be mvoed into the WHERE condition. For example. if you wanted to restrict your query to groups that have COUNT(DZIALU) > 10, say, you would need to put the condition into a HAVING because it acts on the groups, not the individual rows.