How to replace OR operator with UNION operator?

Question

Here is my query:

SELECT 
    h.id,
    h.subject,
    h.body matnF,
    h.amount,
    h.keywords tags,
    h.closed,
    h.author_id author,
    h.AcceptedAnswe         


        

Answer 1

If you are going to get any advantage from using a UNION instead of an OR then make that portion of the query as tiny as it can be, in particular keep the number of columns to a minimum. Do the union before any other joins. I suggest this:

SELECT
      h.id
    , h.subject
    , h.body      AS matnF
    , h.amount
    , h.keywords  AS tags
    , h.closed
    , h.author_id AS author
    , h.AcceptedAnswer
    , h.type
    , h.visibility
    , h.date_time
    , v.value     AS vote_value
    , u.reputation
    , u.user_fname
    , u.user_lname
    , u.avatar
    , h.author_id AS hasUserId
    , (     SELECT COALESCE(SUM(vv.value), 0)
            FROM votes vv
            WHERE h.id = vv.post_id
                  AND vv.table_code = '$this->table_code'
      )  AS total_votes
    , (     SELECT COUNT(1)
            FROM favorites ff
            WHERE h.type = 0 AND h.id = ff.post_id
                  AND ff.table_code = '$this- >table_code'
      ) AS total_favorites
    , CASE
            WHEN h.type = 0 AND f.id IS NOT NULL THEN '2'
            ELSE '3'
      END  AS favorite
    , CASE
            WHEN h.type = 1 THEN '0'
            WHEN h.amount IS NULL OR h.author_id = :user_id2 THEN '1'
            ELSE EXISTS( select 1
                from money m
                where m.user_id = :user_id3 and m.post_id = h.id
               )
      END paid
FROM (
      SELECT q.id , q.author_id , q.visibility , q.type FROM quanda q WHERE q.id = :id1
      UNION
      SELECT q.id , q.author_id , q.visibility , q.type FROM quanda q WHERE q.related = :id2
      ) h
      LEFT JOIN votes v ON h.id = v.post_id
                  AND v.user_id = :user_id4
                  AND v.table_code = '$this->table_code'
      LEFT JOIN favorites f ON h.type = 0
                  AND h.id = f.post_id
                  AND f.user_id = :user_id5
                  AND f.table_code = '$this->table_code'
      LEFT JOIN users u ON h.author_id = u.id
                  AND h.visibility = 1