mysql - mysql 子查询错误

Question

SELECT
  c.cy_id AS cy_id,
  (SELECT 
     c.to_id
   FROM c
   WHERE (c.to_id = se.ar))AS tsn,
  (SELECT
     se.x
   FROM se
   WHERE (c.to_id = se.ar)) AS X,
  (SELECT
     se.y
   FROM se
   WHERE (c.to_id = se.ar)) AS y
FROM (c
   JOIN se)
WHERE (c.to_id = se.ar)

当我在 mysql 中执行时，上面的查询给出错误子查询返回多于一行。但是当我执行以下查询时，它会返回结果。

SELECT
  c.cy_id AS cy_id,
  (SELECT DISTINCT
     c.to_id
   FROM c
   WHERE (c.to_id = se.ar))AS tsn,
  (SELECT
     se.x
   FROM se
   WHERE (c.to_id = se.ar)) AS X,
  (SELECT
     se.y
   FROM se
   WHERE (c.to_id = se.ar)) AS y
FROM (c
   JOIN se)
WHERE (c.to_id = se.ar).

我需要一个理由。

Answer 1

您的 c 表似乎有多行，但它们都具有相同的 to_id 值。如果子查询返回多条记录，您将收到错误。

此外，您的查询似乎不需要任何子查询。试试这个，看看它是否能得到你想要的结果:

SELECT
  c.cy_id,
  c.to_id AS tsn,
  se.x AS X,
  se.y AS y
FROM c
   JOIN se
      ON c.to_id = se.ar