SELECT GROUP查询生成的组数

Question

我尝试了以下查询来选择GROUP BY查询产生的组数：

SELECT count(*)
FROM (
        SELECT count(*)
        FROM MyTable
        WHERE Col2 = 'x'
        GROUP BY Col1
     )

不幸的是，此查询无效：Incorrect syntax near ')'.。

注意：我想得到一行（我已经找到了n次结果的解决方案，其中n =组计数）。

Answer 1

SELECT COUNT(*)
FROM (
    SELECT value = COUNT(*)
    FROM MyTable
    WHERE Col2 = 'x'
    GROUP BY Col1
) е

但我认为 - 您需要尝试此查询 -

SELECT COUNT(DISTINCT Col1)
FROM MyTable
WHERE Col2 = 'x'

Answer 2

SELECT count(*)
FROM (
        SELECT 1 as dummy
        FROM MyTable
        WHERE Col2 = 'x'
        GROUP BY Col1
     ) dt

无需计算子查询中的行数，结果无论如何都会相同。

Answer 3

你必须为列和表添加名称......

SELECT count(*) Total
FROM (
        SELECT count(*) Groups
        FROM MyTable
        WHERE Col2 = 'x'
        GROUP BY Col1
     ) a