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

但是它不工作并且嵌套 SQL 部分中的部分 WHERE b.id = ID 和 WHERE b.another_id = ID 有问题。

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

Answer 1

我想你这里有的叫做相关子查询。它看起来很有前途，似乎只缺少最后几笔(以粗体斜体突出显示):

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 = <b><i>a.</i></b>ID
      UNION 
      SELECT title
      FROM `table_b` AS b
        JOIN table_c ON c.id = b.id
      WHERE b.another_id = <b><i>a.</i></b>ID
  ) AS other_titles 
FROM table_a <b><i>AS a</i></b>

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，并且您不喜欢在子查询中重复它，那么您可能需要重构整个查询，可能像这样:

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