在多列上区分 [英] DISTINCT on multiple columns

问题描述

我有，SELECT DISTINCT (first),second,third FROM table

而且我不仅希望第一个成为DISTINCT，第二个希望成为DISTINCT，而且我希望第三个保持不区分DISTINCT，

AND i want not only the first to be DISTINCT and the second to be DISTINCT to but the third to stay without DISTINCT , i tryed like that.

SELECT DISTINCT (first,second),third FROM table

还有其他几件事，但没有奏效.

And couple more things but didnt worked.

推荐答案

SELECT  m.first, m.second, m.third -- and possibly other columns
FROM    (
        SELECT  DISTINCT  first, second
        FROM    mytable
        ) md
JOIN    mytable m
ON      m.id =
        (
        SELECT  id
        FROM    mytable mi
        WHERE   mi.first = md.first
                AND mi.second = md.second
        ORDER BY
                mi.first, mi.second, mi.third
        LIMIT 1
        )

在(first, second, third)上创建索引以使其快速运行.

Create an index on (first, second, third) for this to work fast.

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