SQL order of operations

sql join optimization where sql-execution-plan

tinkertime · May 18, 2009 · Viewed 10k times · Source

If I run the following SQL query

SELECT * 
FROM A
LEFT JOIN B
ON A.foo=B.foo
WHERE A.date = "Yesterday"

Does the WHERE statement get evaluated before or after the JOIN?

If after, what would be a better way to write this statement so that returns only rows in A from "Yesterday" are joined to B?

Answer

It depends on the database.

On SQL Server, run: SET SHOWPLAN_ALL ON then run the query, you will get an idea of what happens when it runs.