为什么在子查询中不能选择多个列？

Question

我一直在做这个查询

SELECT
    A.*,
    (
        SELECT
            SUM(IF(LENGTH(B.picture) > 0, 1, 0)) AS A_picture_count,
            SUM(IF(LENGTH(B.video) > 0, 1, 0)) AS A_video_count
        FROM B
        WHERE B.A_id = A.id
    )
FROM A

回答我的Operand should contain 1 column(s)而

SELECT
    A.*,
    (
        SELECT
            SUM(IF(LENGTH(B.picture) > 0, 1, 0)) AS A_picture_count
        FROM B
        WHERE B.A_id = A.id
    )
FROM A

完美地工作。 为什么MariaDB不会让我子查询超过1列？ 我想念什么吗？

---

正如这个问题（ MySQL-Operand应该包含1列 ）所暗示的那样，

您可能会回答我应该这样

SELECT t1.*, sq.*
FROM table1 t1,
   (SELECT a,b,c FROM table2 ...) sq
WHERE ...

或使用联接（但是我的查询要复杂得多，并且我的许多SUMS和COUNTS与我的GROUP BY混为一谈）。

我的问题更多是关于“为什么”而不是“如何”。

---

我的最终解决方案（不是最佳方案）：

SELECT
    A.*,
    (
        SELECT
            SUM(IF(LENGTH(B.picture) > 0, 1, 0)) AS A_picture_count,
        FROM B
        WHERE B.A_id = A.id
    )
    (
        SELECT
            SUM(IF(LENGTH(B.video) > 0, 1, 0)) AS A_video_count
        FROM B
        WHERE B.A_id = A.id
    )
FROM A

Answer 1

令人发指的是将两个值都编码为一个变量。 假设两个数字都不超过1000：

SELECT
    A.*,
    (
        SELECT
         1000 * SUM(IF(LENGTH(B.picture) > 0, 1, 0)) +
                SUM(IF(LENGTH(B.video) > 0, 1, 0)) AS A_pv_count
        FROM B
        WHERE B.A_id = A.id
    )
FROM A

您可以使用/ 1000和％1000返回组件

SELECT floor(A_pv_count / 1000) AS A_picture_count,
            (A_pv_count % 1000) AS A_video_count
FROM
(SELECT
    A.*,
    (
        SELECT
         1000 * SUM(IF(LENGTH(B.picture) > 0, 1, 0)) +
                SUM(IF(LENGTH(B.video) > 0, 1, 0)) AS A_pv_count
        FROM B
        WHERE B.A_id = A.id
    )
 FROM A) AS AW

Answer 2

IMO No Database应该允许您像这样在( )选择两列。 显然，您不能select col1, (col2_ok), (multi_columns_HOW_ON_EARTH?)

一种解决方案应该使用这样的tmp表（已搜索但MariaDB不支持cte ）：

select A.*, tmp.A_picture_count, tmp.A_video_count
from A
inner join (SELECT A.id,
                SUM(IF(LENGTH(B.picture) > 0, 1, 0)) AS A_picture_count,
                SUM(IF(LENGTH(B.video) > 0, 1, 0)) AS A_video_count
            FROM A
            inner join B
            on B.A_id = A.id
            group by A.id
            ) tmp
on A.id = tmp.id;