如何将SELECT语句用于JOIN子句？

Question

我有一个包含动态查询的变量。事情是这样的：

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

我也有这个疑问：

SELECT code FROM mytable WHERE id = :id 

现在我想join上面的查询与动态查询。这是我的尝试：

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 
*/ 

它的工作原理。但是，大量数据需要很长时间。我怎样才能让它更快？

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 

只需将：id检查添加到内部querie4st以限制所选数据量。

Answer 2

您可以使用下面的查询。

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