mysql - 多列上的 DISTINCT

Question

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

而且我不仅希望第一个是 DISTINCT，第二个是 DISTINCT，而且第三个没有 DISTINCT，我试过了。

SELECT DISTINCT (first,second),third FROM table

还有更多的事情，但没有奏效。

Answer 1

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。