Question

假设我有这样的查询：

SELECT * FROM (
  SELECT 'a' AS a, '1' AS b
UNION 
  SELECT 'a' AS a, '2' AS b
UNION
  SELECT 'b' AS a, '1' AS b) AS a 
GROUP BY a.a

在这种情况下，“a”.b是1,2的汇总，而“b”.b只是1的汇总。

如何只选择“a”？

问题更新更清晰：

让我们采用这个非常相似的查询：

SELECT *, GROUP_CONCAT(b) FROM (
  SELECT 'a' AS a, '1' AS b
UNION 
  SELECT 'a' AS a, '2' AS b
UNION
  SELECT 'a' AS a, '3' AS b
UNION
  SELECT 'b' AS a, '1' AS b
UNION
  SELECT 'b' AS a, '2' AS b
) AS a 
GROUP BY a.a

现在a.b是1,2,3而b.b是1,2：

我想在它有1,2和3的基础上选择一个。

Answer 1

SELECT  a
FROM    (
        SELECT 'a' AS a, '1' AS b
        UNION 
        SELECT 'a' AS a, '2' AS b
        UNION
        SELECT 'b' AS a, '1' AS b
        ) AS a
GROUP BY
        a
HAVING  COUNT(*) > 1

请注意，您使用的*语法是MySQL的扩展名，不应该像这样使用。

在这种情况下，

a.b不是聚合：它将是来自组'a'的随机记录。

Answer 2

尝试：

SELECT a.a,COUNT(*) AS CountOf FROM (
  SELECT 'a' AS a, '1' AS b
UNION 
  SELECT 'a' AS a, '2' AS b
UNION
  SELECT 'b' AS a, '1' AS b) AS a 
GROUP BY a.a
HAVING  COUNT(*) > 1

输出：

a    CountOf
---- -----------
a    2

(1 row(s) affected)

编辑基于OP的评论：

这不会出现a.b. 1,2,3和b.b是1,2

试试这个：

SELECT TOP 1 a.a,COUNT(*) AS CountOf FROM (
  SELECT 'a' AS a, '1' AS b
UNION 
  SELECT 'a' AS a, '2' AS b
UNION 
  SELECT 'a' AS a, '3' AS b
UNION 
  SELECT 'b' AS a, '1' AS b
UNION
  SELECT 'b' AS a, '2' AS b) AS a 
GROUP BY a.a
ORDER BY 2 DESC

输出：

a    CountOf
---- -----------
a    3

(1 row(s) affected)

Answer 3

以下似乎对我有用：

SELECT a,b FROM (
    SELECT a,b FROM (
      SELECT 'a' AS a, '1' AS b
    UNION 
      SELECT 'a' AS a, '2' AS b
    UNION
      SELECT 'a' AS a, '3' AS b
    UNION
      SELECT 'b' AS a, '1' AS b
    UNION
      SELECT 'b' AS a, '2' AS b
    ) AS a 
) b WHERE b.b IN ({x1},{x2}...)
GROUP BY b.a HAVING COUNT(b.b) = 2

其中x1等是我想要限制的“b”的值。

例如，在这种情况下

WHERE b.b IN (1,2,3)

将返回“a”

WHERE b.b IN (1,2)

将返回“a”和“b”

和

WHERE b.b IN (2,3)

只会返回“a”

我不确定为什么会这样，但似乎。

选择何时聚合列具有两个或更多项

3 个答案: