MySQL Join clause vs WHERE clause

问题:

What's the difference in a clause done the two following ways?

SELECT * FROM table1 INNER JOIN table2 ON (
    table2.col1 = table1.col2 AND
    table2.member_id = 4
)

I've compared them both with basic queries and EXPLAIN EXTENDED and don't see a difference. I'm wondering if someone here has discovered a difference in a more complex/processing intensive envornment.

SELECT * FROM table1 INNER JOIN table2 ON (
    table2.col1 = table1.col2
)
WHERE table2.member_id = 4

回答1:

With an INNER join the two approaches give identical results and should produce the same query plan.

However there is a semantic difference between a JOIN (which describes a relationship between two tables) and a WHERE clause (which removes rows from the result set). This semantic difference should tell you which one to use. While it makes no difference to the result or to the performance, choosing the right syntax will help other readers of your code understand it more quickly.

Note that there can be a difference if you use an outer join instead of an inner join. For example, if you change INNER to LEFT and the join condition fails you would still get a row if you used the first method but it would be filtered away if you used the second method (because NULL is not equal to 4).

回答2:

If you are trying to optimize and know your data, by adding the clause "STRAIGHT_JOIN" can tremendously improve performance. You have an inner join ON... So, just to confirm, you want only records where table1 and table2 are joined, but only for table 2 member ID = some value.. in this case 4.

I would change the query to have table 2 as the primary table of the select as it has an explicit "member_id" that could be optimized by an index to limit rows, then joining to table 1 like

select STRAIGHT_JOIN
      t1.*
   from
      table2 t2,
      table1 t1
   where 
         t2.member_id = 4
      and t2.col1 = t1.col2

So the query would pre-qualify only the member_id = 4 records, then match between table 1 and 2. So if table 2 had 50,000 records and table 1 had 400,000 records, having table2 listed first will be processed first. Limiting the ID = 4 even less, and even less when joined to table1.

I know for a fact the straight_join works as I've implemented it many times dealing with gov't data of 14+ million records linking to over 15 lookup tables where the engine got confused trying to think for me on the critical table. One such query was taking 24+ hours before hanging... Adding the "STRAIGHT_JOIN" and prioritizing what the "primary" table was in the query dropped it to a final correct result set in under 2 hours.

回答3:

There's not really much of a difference in the situation you describe; in a situation with multiple complex joins, my understanding is that the first is somewhat preferential, as it will reduce the complexity somewhat; that said, it's going to be a small difference. Overall, you shouldn't notice much of a difference in most if not all situations.

回答4:

With an inner join, it makes almost* no difference; if you switch to outer join, all the difference in the world.

*I say "almost" because optimizers are quirky beasts and it isn't impossible that under some circumstances, it might do a better job optimizing the former or the latter. Do not attempt to take advantage of this behavior.