如何在SELECT語句中使用JOIN子句？

Question

我有一個包含動態查詢的變量。 像這樣：

$query = "SELECT id, subject FROM post1
          UNION ALL
          SELECT id, subject FROM post2
          UNION ALL
          SELECT id, subject FROM post3";

我也有這個查詢：

SELECT code FROM mytable WHERE id = :id

現在我想join上述與動態查詢的查詢。 這是我的嘗試：

SELECT t1.code t2.subject FROM mytable t1 
   LEFT JOIN ($query) t2 ON t1.col = t2.id
WHERE t1.id = :id

/*
SELECT t1.code t2.subject FROM mytable t1
   LEFT JOIN (SELECT id, subject FROM post1
               UNION ALL
              SELECT id, subject FROM post2
               UNION ALL
              SELECT id, subject FROM post3) t2 ON t1.col = t2.id
WHERE t1.id = :id
*/

有用。 但是海量數據需要大量時間。 我怎樣才能使其更快？

Answer 1

SELECT t1.code t2.subject FROM mytable t1
   LEFT JOIN (SELECT id, subject FROM post1 
              JOIN mytable tt1 ON tt1.col = post1.id AND tt1.id=:id
               UNION ALL
              SELECT id, subject FROM post2
              JOIN mytable tt2 ON tt2.col = post2.id AND tt2.id=:id
               UNION ALL
              SELECT id, subject FROM post3
              JOIN mytable tt3 ON tt3.col = post3.id AND tt3.id=:id) t2 ON t1.col = t2.id
WHERE t1.id = :id

只需將：id檢查添加到內部查詢中即可限制所選數據量。

Answer 2

您可以使用以下查詢。

SELECT t1.code, t2.subject FROM (SELECT code, col FROM mytable WHERE id = :id ) t1 
 LEFT JOIN ($query) t2 ON t1.col = t2.id