Need suggestion formore pretty sql query - mysql

Question

I have this query, it does what it is supposed to do.
It finds all records in table 1that have a match in table 2, and assign a pseudo column value 1.
Then it union a query that retreive all records in table 1 that does not have a match in table 2, and assign a pseudo column value 0.
This works, but I am sure it is possible to do it in a more effective way.
SELECT aplac.*, 1 AS selected
FROM aux_placements aplac
JOIN product_placements pplac
ON pplac.placement_id = aplac.placement_id
WHERE pplac.product_id = 1
UNION
SELECT distinct aplac.*, 0 AS selected
FROM aux_placements aplac
WHERE placement_id NOT IN
(SELECT aplac.placement_id
FROM aux_placements aplac
JOIN product_placements pplac
ON pplac.placement_id = aplac.placement_id
WHERE pplac.product_id = 1)
I was thinking something like this might work:
SELECT aplac.*,
CASE WHEN aplac.placement_id IS NULL
THEN 0
ELSE 1
END AS selected
FROM aux_placements aplac
LEFT OUTER JOIN product_placements pplac
ON pplac.placement_id = aplac.placement_id
WHERE pplac.product_id = 1;
Any suggestion is appreciated.
Thanks in advance
EDIT: sqlfiddle is avaliable here: http://sqlfiddle.com/#!2/57013a/1

Answer 1

You want the product id selection to be part of the join condition - change WHERE to AND.
You want to check whether the join found a matching record in the second table - change aplac.placement_id to pplac.placement_id.
So the next query will work for you:
SELECT aplac.*,
CASE WHEN pplac.placement_id IS NULL
THEN 0
ELSE 1
END AS selected
FROM aux_placements aplac
LEFT OUTER JOIN product_placements pplac
ON pplac.placement_id = aplac.placement_id
AND pplac.product_id = 1;