MySQL实现分组排序（取第一或最后）

按order_num分组取时间最早的（LAST_UPDATE_DATE asc），若要取最晚的改为desc

select ORDER_NUM,LAST_UPDATE_DATE,OPERATOR,rank from
(
    select t.ORDER_NUM
        ,t.LAST_UPDATE_DATE
        ,t.OPERATOR
        ,@rownum:=@rownum+1 ,
        if(@order_num=t.ORDER_NUM,@rank:=@rank+1,@rank:=1) as rank,
        @order_num:=t.ORDER_NUM
    from
    (
        select ORDER_NUM,LAST_UPDATE_DATE,OPERATOR from table_name 
        order by ORDER_NUM asc,LAST_UPDATE_DATE asc
    ) t
    ,(select @rownum :=0 , @order_num := null ,@rank:=0) a
) result
order by ORDER_NUM desc

最近经常处理mysql字符串，突然发现一种不一样的思路，也可以实现分组排序取第一或最后：

select ORDER_NUM
    ,case when locate('|',OPERATOR) > 0 then SUBSTR(OPERATOR,1,INSTR(OPERATOR,'|')-1) else OPERATOR end
from
(
    SELECT ORDER_NUM
        ,group_concat(DISTINCT OPERATOR ORDER BY LAST_UPDATE_DATE SEPARATOR '|') AS OPERATOR
    FROM table_name
    GROUP BY ORDER_NUM
) result

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最近看别人的面试题遇到一个需要按列的大小取最大的，在此记录下：

select id, greatest(score1,score2,score3) as score from score;

从score表中取出每个id最大的score（取最小则用least）