MySQL与另一个表中的ON条件

Question

Let's say that I have:

SELECT * FROM a
LEFT OUTER JOIN b ON b.a_id = a.id
LEFT OUTER JOIN c ON b.c_id = c.id

Now what I want to do is to select b's that are assigned to c that is eg active (c.active = 1). How can I do that with ON?

Note that I can't use WHERE after the whole query above, because I want a's to be returned even if 0 b's are found.

Answer 1

Did you try something like this?

SELECT * FROM a LEFT OUTER JOIN b ON b.a_id = a.id AND c.active = "1" LEFT OUTER JOIN c ON b.c_id = c.id AND c.active = "1"

Answer 2

Just to make sure, I understood the question: You want all rows where either c.active equals 1 or where there is no entry in b or c, right?

It's a bit lengthy but this seems to work:

SELECT * FROM a 
LEFT OUTER JOIN b ON a.aid = b.aid 
LEFT OUTER JOIN c ON b.bid = c.bid 
WHERE a.aid NOT IN (
    SELECT a.aid FROM a 
    INNER JOIN b ON a.aid = b.aid 
    INNER JOIN c ON b.bid = c.bid 
    WHERE NOT c.active
);

I could also imagine a solution using UNION

Answer 3

Which rows are returned in each case... Note that one case is different:
FROM b JOIN c ON ... AND c.active=1 -- rows in both tables exist and active
FROM b JOIN c ON ... WHERE c.active=1 -- ditto
FROM b LEFT JOIN c ON ... WHERE c.active=1 -- ditto
FROM b LEFT JOIN c ON ... AND c.active=1 -- all b's, but NULLs for inactive/missing c's
(Caveat: I am not sure I got the cases correct; just keep in mind that ON and WHERE are not always interchangeable.)

When mixing JOIN and LEFT JOIN, you may need to add parentheses:
FROM a JOIN ( b LEFT JOIN c ON... ) ON ... WHERE ...
FROM ( a JOIN b ON ... ) LEFT JOIN c ON... WHERE ...