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

In MySQL:

If we have two tables:

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

and:

meta
comment_key    |    value
=========================
1              |    1
2              |    1
3              |    2
4              |    1

I want to get the comments from the comment table that have a corresponding comment_key in the meta table that have a specific value (the value column in the meta table).

For example, I'd like to select all the rows from the comment table that have a value of 1 in the meta table:

I'd expect these results:

key    |    value
=================
1      |    foo
2      |    bar
4      |    barfoo

And if I were to select all the rows from the comment table that have a value of 2 in the meta table:

I'd expect this result:

key    |    value
=================
3      |    foobar

I really hope someone can help, thank you all in advance!

I think I need to do a join? Any pointers would be great, and if at all possible, a short explanation so I can work out where I was going wrong -> so I'll know for next time!

Solution

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.)