Suppose you have a table codes :
(
game_id INT,
code TEXT,
used BOOL NOT NULL DEFAULT 'f',
prize ...
...
PRIMARY KEY (game_id, code)
)
Just UPDATE codes SET used='t' WHERE used='f' AND game_id=... AND
code=...
Then check the rowcount : if one row was updated, the code was not
used yet. If no row was updated, the code either did not exist, or was
already used.
You can use a stored procedure with exceptions no?
Try this:
http://www.postgresql.org/docs/8.1/interactive/plpgsql-control-structures.html#PLPGSQL-UPSERT-EXAMPLE
Chris
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