MySQL 排名統計(常用功能函數)

select actor_id,@curr_cnt:=cnt as cnt ,
@rank:=IF(@prev_cnt!=@curr_cnt,@rank+1,@rank) as rank,
@prev_cnt:=@curr_cnt as dummy
from(
select actor_id,count(*) as cnt from film_actor 
group by actor_id ORDER BY cnt desc
) as der,
(select @curr_cnt:=0,@prev_cnt:=0,@rank:=0 ) as t -- 實際上相當於賦值操作

-- 排名，考慮並列情況
SELECT @rownum:=@rownum+1 AS rownum,IF(@total = cnt ,@rank ,@rank :=@rownum) AS rank ,
@total := cnt as total1, der.* FROM
(
select actor_id,count(*) as cnt from film_actor 
group by actor_id ORDER BY cnt desc
) as der,
(    SELECT    @rank := 0 ,@rownum := 0 ,@total := NULL    ) as t

 

-- c100w 目標表  c10w 源表    對應關系 c10w.id=c100w.id
update c100w,c10w set c100w.grender=c10w.grender where c10w.id=c100w.id   -- 10w 2.083s

 

常見性能優化場景：

1、多表連接查詢很慢，而且不論結果多少，查詢耗時差不多。可能原因缺少索引導致全表掃碼 ，考慮索引優化

查詢缺少索引時，減少表連接看耗時是否減少，再排查連接表是否有索引。