Question

Just now I needed to do something like the following query, and was surprised that it actually worked as intented. However I cannot find any documentation on it so I’m a little afraid about unknown side effects of this. Here’s what I wrote:

select * from Table1 where (col1, col2) in (select col3, col4 from Table2)

This seems to be matching a pair of columns against a list of column-pairs. Is this how it is supposed to work, or can I expect some nasty surprises down the road?

Answer 1

It seems there is a page in the MySQL manual about that syntax : 12.2.9.5. Row Subqueries

One of the given examples is this one (quoting) :

SELECT column1,column2,column3
       FROM t1
       WHERE (column1,column2,column3) IN
             (SELECT column1,column2,column3 FROM t2);

Which is quite similar to your query.

So I suppose it is officially supported.

The same page says (quoting) :

Row constructors are legal in other
contexts as well. For example, the
following two statements are
semantically equivalent (although the
first one cannot be optimized until
MySQL 5.0.26):

SELECT * FROM t1 WHERE (column1,column2) = (1,1);
SELECT * FROM t1 WHERE column1 = 1 AND column2 = 1;

So, even if the syntax you proposed is valid, the second one might be better, if you can use it, at least — and more easy to read / understand 😉

And you’ll find plenty other stuff about subqueries from the page 12.2.9. Subquery Syntax

(Yeah, wasn’t easy to find ^^ )