MySQL - 在同一SQL的嵌套SQL语句中使用select字段值

Question

我有以下SQL查询

SELECT 
    a.id AS ID, a.title, 
    (
        SELECT Group_Concat( title ) 
        FROM (
            SELECT title
            FROM `table_b` AS b
            JOIN table_c ON c.id = b.id
            WHERE b.id = ID
            UNION 
            SELECT title
            FROM `table_b` AS b
            JOIN table_c ON c.id = b.id
            WHERE b.another_id = ID
    ) AS other_titles 
FROM table_a

但是它没有工作，并且在嵌套SQL部分中有部件WHERE b.id = ID和WHERE b.another_id = ID的问题。

如何在嵌套选择中使用第一个选择（ SELECT a.id AS ID ）中的SELECT a.id AS ID ？

Answer 1

我认为你在这里所谓的相关子查询 。 它看起来非常有前途，似乎只缺少几个最终笔画（用粗体斜体突出显示）：

SELECT
  a.id AS ID,
  a.title, 
  (
    SELECT Group_Concat( title ) 
    FROM (
      SELECT title
      FROM `table_b` AS b
        JOIN table_c ON c.id = b.id
      WHERE b.id = ID
      UNION 
      SELECT title
      FROM `table_b` AS b
        JOIN table_c ON c.id = b.id
      WHERE b.another_id = ID
  ) AS other_titles 
FROM table_a 

请记住，您在子查询中实际引用的是a.id ，而不是ID别名。 （你不能在那里引用别名。）

如果a.id是一个复杂的表达式，并且你不太乐意在子查询中重复它，那么你可能需要重构整个查询，可能是这样的：

SELECT
  x.ID,
  x.title,
  (
    SELECT Group_Concat( title ) 
    FROM (
      SELECT title
      FROM `table_b` AS b
        JOIN table_c ON c.id = b.id
      WHERE b.id = x.ID
      UNION 
      SELECT title
      FROM `table_b` AS b
        JOIN table_c ON c.id = b.id
      WHERE b.another_id = x.ID
  ) AS other_titles 
FROM (
  SELECT
    a.id AS ID,
    a.title,
  FROM table_a AS a
) x

Answer 2

您不能在FROM子句中选择值。 您只能使用变量来动态设置表名。