MySQL：从一栏中选择多个最大值

Question

假设我有这张桌子：

mytable

 +------+-------+ | type | count | +------+-------+ | red | 4 | | blue | 3 | | red | 2 | | blue | 7 | +------+-------+ 

现在我要回来：

 +--------+---------+ | maxRed | maxBlue | +--------+---------+ | 4 | 7 | +--------+---------+ 

我怎样才能做到这一点？

这就是我尝试过的

SELECT MAX(count) as maxRed, 0 as maxBlue FROM mytable WHERE type='red'

UNION

SELECT 0 as maxRed, MAX(count) as maxBlue FROM mytable WHERE type='blue'

但这并不完全有效，只会产生结果

 +--------+---------+ | maxRed | maxBlue | +--------+---------+ | 4 | 0 | | 0 | 7 | +--------+---------+ 

Answer 1

为了使查询正常工作，您只需要用另一个查询将其包装即可：

SELECT MAX(maxRed) as maxRed, MAX(maxBlue) as maxBlue
FROM(
    SELECT MAX(count) as maxRed, 0 as maxBlue FROM mytable WHERE type='red'
    UNION
    SELECT 0 as maxRed, MAX(count) as maxBlue FROM mytable WHERE type='blue') t

一种更优雅的方法是条件聚合：

SELECT MAX(CASE WHEN t.type = 'red' THEN t.count END) as maxRed,
       MAX(CASE WHEN t.type = 'blue' THEN t.count END) as maxBlue
FROM mytable t
WHERE t.type IN('blue','red')

Answer 2

这是怎么做的：

SELECT
MAX(CASE WHEN type = 'red' THEN `count` ELSE NULL END) AS maxRed,
MAX(CASE WHEN type = 'blue' THEN `count` ELSE NULL END) AS maxBlue
FROM
mytable
WHERE type IN ('red', 'blue');

Answer 3

SELECT type,MAX(count) FROM mytable group by type 

这将以垂直而不是水平的方式返回类型和最大计数，并且可扩展为n个记录。

如果水平数据无关紧要，则可以使用上面建议的答案来解决用例，但对于每个不同的数据值，则需要用例。

Answer 4

编辑由于下面的注释我在真实数据上进行了测试，因此对mytable进行索引时，对于大型表，我的版本要快一些。 对于那些争论的人，请测试一下自己并报告。

 select 
(select max(count) from mytable  WHERE type='red') aa,
(select max(count) from mytable  WHERE type='blue') bb