低效的子查询

select a.user_name,a.over,(select over from user2 b where a.user_name=b.user_name) as over2 from user1 a;

使用join优化后的子查询：

select a.user_name,a.over,b.over as over2 from user1 a left join user2 b on a.user_name=b.user_name;

使用join + having优化聚合子查询:

select a.user_name,b.timestr,b.kills from user1 a

join user_kills b on a.id = b.user_id

join user_kills c on c.user_id = b.user_id

group by a.user_name,btimestr,b.kills

having b.kills = max(c.kills);

分类聚合方式查询每一个用户某一个字段数据最大的两条数据：

select d.user_name ,c.ctimestr,kills from

(select user_id ,timestr ,kills ,(

select count(*) from user_kills b where b.user_id = a.user_id and a.kills <= b.kills) as cnt

from user_kills a group by user_id,timestr,kills) c

join user1 d on c.user_id = d.id where cnt <= 2

select * from (

select sum(kills) as '孙悟空' from user1 a

join user_kills b

on a.id = b.use_id and a.user_name='孙悟空'

) a cross join (

select sum(kills) as '猪八戒' from user1 a

join user_kills b

on a.id = b.use_id and a.user_name='猪八戒'

) b cross join(

select sum(kills) as '沙僧' from user1 a

join user_kills b

on a.id = b.use_id and a.user_name='沙僧'

) c

select

sum(case when user_name=' 孙悟空' then kills end) as '孙悟空',

sum(case when user_name='猪八戒' then kills end) as '猪八戒',

sum(case when user_name='沙僧' then kills end) as '沙僧'

from user1 a join use_kills b

on a.id = b.user_id;

select user_name,

replace(substring(substring_index(mobile,','a.id),char_length(substring_index(mobile,',',a.id-1))+1),',','') as mobile --截取字符串

from tb_sequence as a

cross join(

select user_name,

concat(mobile,',') as mobile, --在mobile结尾增加逗号

length(mobile)-length(replace(mobile,',',''))+1 as size --通过mobile总长度-排除逗号后的长度+1计算得出总段数

from user1 as b

) as b on a.id<=b.size

SELECT user_name,'arms' as equipment, arms FROM

user1 a JOIN user1_equipment b ON a.id=b.user_id

UNION ALL

SELECT user_name,'clothing' AS equipment,clothing FROM user1 a JOIN user1_equipment b ON a.id=b.user_id

UNION ALL

SELECT user_name,'shoe' AS equipment,shoe FROM user1 a JOIN user1_equipment b ON a.id=b.user_id