How to create fast database queries

Archive for October, 2010

Correlated LEFT JOIN in a subquery

with one comment

Answering questions asked on the site.

Beren asks:

In my database, I store the information about the game matches.

A match can be played between two players (mostly) or, occasionally, between two teams of 2, 3 or 4 players.

I store the matches in a table like this:

id player1 player2 type

, where type is ENUM(player, team).

If type is player, the ids of players are stored in the record; if type is team, those of teams are stored.

Now, the tricky part. For a given game, I need to select two lists of players from both sides, comma separated, be they single players or members of a team.

Are two separate queries required for this?

This of course can be easily done using a single query.

Let's make a sample table and see:
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Written by Quassnoi

October 20th, 2010 at 11:00 pm

Posted in MySQL