Select rows from a table where row in another table with same id has a particular value in another column

Question

In MySQL:

If we have two tables:

comments
key    |    value
=================
1      |    foo
2      |    bar
3      |    foobar
4      |    barfoo
         


        

Answer 1

I actually wouldn't recommend a JOIN for this — or rather, I'd recommend a "semijoin", which is a relational-algebra concept not directly expressed in SQL. A semijoin is essentially a join where you want to retrieve records from only one table, but with the proviso that they have corresponding records in a different table.

In SQL notation, this concept is expressed indirectly, by using an IN clause, with a subquery:

SELECT key, value
  FROM comments
 WHERE key IN
        ( SELECT comment_key
            FROM meta
           WHERE value = 1
        )
;

(MySQL will actually end up translating that back into a semijoin internally — essentially a sort of degenerate inner-join — but the IN clause is the natural way to express it in raw SQL.)