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将子查询改为join写法时要注意表中关系是不是一对多或多对多，这种情况下要对查询结果使用distinct去重。

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select a.user_name,b.timestr,kills from user1 a join user_kills b on

a.id=b.user_id join(select user_id,max(kills) as cnt from user_kills group by user_id)c on

b.user_id=c.user_id and b.kills=c.cnt;

mysql中独有的列过滤方式：多列过滤

select a.user_name,b.timestr,kills

from user1 a

join user_kills b on a.id=b.user_id

where (b.user_id,b.kills) in (

select user_id,max(kills)

from user_kills

group by user_id

)；