从子查询中获取三列中的1列

Question

在我的查询

中

SELECT a.id, a.rev, a.content,
(
  SELECT 
  EXISTS(SELECT id FROM `docs` WHERE id = 1), 
  EXISTS(SELECT id FROM `docs` WHERE id = 2), 
  EXISTS(SELECT id FROM `docs` WHERE id = 3)
) AS ex
FROM `docs` a

我希望它返回像逗号分隔的子查询值

id rev content ex
1  1   ....... 1,1,0

但是我不断获得Operand should contain 1 column(s)，当我想从一个子查询中返回单列中的多列值时，我该如何处理这个问题？

我得到的是我提到的error，但是我想要得到的是

id  rev     content                                              ex
1   1       The earth is flat                                    1,1,0
1   2       The earth is flat and rests on a bull's horn         1,1,0
1   3       The earth is like a ball.                            1,1,0
2   1       One hundred angels can dance on the head of a pin    1,1,0

Answer 1

我相信你想要 CONCAT：

小提琴示例

SELECT a.id, a.rev, a.content,
       CONCAT(CAST(EXISTS(SELECT id FROM `docs` WHERE id = 1) as char(1)),',',
              CAST(EXISTS(SELECT id FROM `docs` WHERE id = 2) as char(1)),',',
              CAST(EXISTS(SELECT id FROM `docs` WHERE id = 3) as char(1))
       ) AS ex
FROM `docs` a