WHERE clause before INNER JOIN

Question

If I have

SELECT * FROM Table1 t1 
LEFT JOIN Table2 t2 ON t1.id = t2.id 
WHERE t1.user=\'bob\';

Does the WHERE clause run afte

Answer 1

The where clause will be executed before the join so that it doesn't join unnecessary records. So your code is fine the way it is.

Answer 2

you can do

SELECT * 
    FROM Table1 t1 
    LEFT JOIN Table2 t2
        ON t1.id=t2.id AND t1.user='bob';

Answer 3

In my experience in a left join you cannot exclude records in the 'left' (t1) table in the ON-statement since - by definition - all t1 records will be included. The where statement does work as it will be applied to the result of the join afterwards.

I do not exactly know what you want to achieve but most probably an inner join suits your needs as well and then you can add the t1.user='bob' condition to the ON-statement.

But if Mosty Mostacho is correct, the location (WHERE vs ON) of the condition is not relevant for speed of execution.

Answer 4

You should just add t1.user='bob' condition to ON clause before other condition and it will be evaluated first:

SELECT * FROM Table1 t1 
LEFT JOIN Table2 t2
ON t1.user='bob' AND t1.id = t2.id;

Answer 5

What you may use is table expression after FROM like this:

SELECT *
FROM (SELECT
        id
    FROM Table1
    WHERE user = 'bob') AS t1
LEFT JOIN Table2 t2
    ON t1.id = t2.id

Answer 6

RIGHT JOIN was the solution:

SELECT cars.manufacturer, cars.year FROM cars 
RIGHT JOIN (SELECT m.manufacturer FROM cars AS m ORDER BY m.year DESC LIMIT 3) subq 
ON cars.manufacturer=subq.manufacturer

Haven't put it through the full rigors yet, but seems to work.