将具有多个值的ONE行转换为具有单个值的MULTIPLE列

Question

我有一个mysql查询

SELECT SUM(idHit = 122791), SUM(idHit = 950521) FROM hits

返回两个值，而不是只有一行有两列：

5, 7

使用mysql语法，我想在两列中获取两个值？像

5
7

这一切，我认为有必要将sql查询转换为子查询，但不确定。请帮助。

Answer 1

使用GROUP BY：

SELECT idHit, COUNT(*)
FROM t
WHERE idHit IN (122791, 950521)
GROUP BY idHit;

如果您遇到两者可能不存在的问题，那么我建议left join：

select ids.idhit, count(t.idhit)
from (select 122791 as idhit union all select 950521) ids left join
     t
     on t.idhit = ids.idhit
group by ids.idhit;

union all是一种有趣的方法。我会把它说成：

SELECT 122791, COUNT(*) FROM hits WHERE idHit = 122791
UNION ALL
SELECT 950521, COUNT(*) FROM hits WHERE idHit = 950521;

仅注意此标准SQL，但如果索引可用，则可以使用索引。

Answer 2

你可以用UNION

来做

SELECT SUM(idHit = 122791) FROM hits
UNION ALL
SELECT SUM(idHit = 950521) FROM hits