Union as sub query MySQL

Question

I\'m wanting to optimize a query using a union as a sub query. Im not really sure how to construct the query though. I\'m using MYSQL 5

Here is the original query:

Answer 1

SELECT  Parts.id
FROM    (
        SELECT  parts_id
        FROM    Parts_Category
        WHERE   Parts_Category.category_id = '508'
        UNION
        SELECT  parts_id
        FROM    Parts_Category
        WHERE   Parts_Category.main_category_id = '508'
        ) pc
JOIN    Parts
ON      parts.id = pc.parts_id
        AND Parts.status = 'A'
LEFT JOIN
        Image
ON      image.id = parts.image_id

Note that MySQL can use Index Merge and you can rewrite your query as this:

SELECT  Parts.id
FROM    (
        SELECT  DISTINCT parts_id
        FROM    Parts_Category
        WHERE   Parts_Category.category_id = '508'
                OR Parts_Category.main_category_id = '508'
        ) pc
JOIN    Parts
ON      parts.id = pc.parts_id
        AND Parts.status = 'A'
LEFT JOIN
        Image
ON      image.id = parts.image_id

, which will be more efficient if you have the following indexes:

Parts_Category (category_id, parts_id)
Parts_Category (main_category_id, parts_id)