MYSQL子查询和嵌套查询优化实例解析

查询游戏历史成绩最高分前100

sql代码

select ps.* from cdb_playsgame ps where ps.credits=(select max(credits)  
from cdb_playsgame ps1  
where ps.uid=ps1.uid and ps.gametag=ps1.gametag) and ps.gametag='yeti3'  
group by ps.uid order by ps.credits desc limit 100; 

sql代码

select ps.*  
from cdb_playsgame ps,(select ps1.uid, ps1.gametag, max(credits) as credits 
from cdb_playsgame ps1 group by uid,gametag) t 
where ps.credits=t.credits and ps.uid=t.uid and ps.gametag=t.gametag and ps.gametag='yeti3'  
group by ps.uid order by ps.credits desc limit 100; 

执行时间仅为0.22秒，比原来的25秒提高了10000倍

查询当天游戏最好成绩

sql代码

 select ps. * , mf. * , m.username 
from cdb_playsgame ps 
left join cdb_memberfields mf on mf.uid = ps.uid 
left join cdb_members m on m.uid = ps.uid 
where ps.gametag = 'chuansj' 
and from_unixtime( ps.dateline, '%y%m%d' ) = '20081008' 
and ps.credits = ( 
select max( ps1.credits ) 
from cdb_playsgame ps1 
where ps.uid = ps1.uid 
and ps1.gametag = 'chuansj' 
and from_unixtime( ps1.dateline, '%y%m%d' ) = '20081008' ) 
group by ps.uid 
order by credits desc 
limit 0 , 50  

像查询里：

and ps.credits=(select max(ps1.credits)  
 from {$tablepre}playsgame ps1 where ps.uid=ps1.uid and ps1.gametag = '$game'  
 and from_unixtime(ps1.dateline, '%y%m%d') = '$todaytime' ) 

特别消耗时间

另外，像：

from_unixtime(ps1.dateline, '%y%m%d') = '$todaytime' 

这样的语句会导致索引无效，因为对每个dataline的值都需要用函数计算一遍，需要调整为：

sql代码

and ps1.dateline >= unix_timestamp('$todaytime')  

//更改后
sql代码

 select ps. * , mf. * , m.username 
from cdb_playsgame ps, cdb_memberfields mf, cdb_members m, ( 
 
select ps1.uid, max( ps1.credits ) as credits 
from cdb_playsgame ps1 
where ps1.gametag = 'chuansj' 
and ps1.dateline >= unix_timestamp( '20081008' ) 
group by ps1.uid 
) as t 
where mf.uid = ps.uid 
and m.uid = ps.uid 
and ps.gametag = 'chuansj' 
and ps.credits = t.credits 
and ps.uid = t.uid 
group by ps.uid 
order by credits desc 
limit 0 , 50  

对于每个球员，找出球员号码，名字以及他所引起的罚款的号码，但只是针对那些至少有两次罚款的球员。

更紧凑的查询，在from子句中放置一个子查询。

sql代码

select playerno,name,number 
from (select playerno,name, 
       (select count(*) 
       from penalties 
       where penalties.playerno = 
          players.playerno) 
       as number 
    from plyers) as pn 
where number>=2 

from子句中的子查询决定了每个球员的号码，名字和罚款的编号。接下来，这个号码变成了中间结果中的一列。然后指定了一个条件（number>=2）;最后，获取select子句中的列。

总结

以上就是本文关于mysql子查询和嵌套查询优化实例解析的全部内容，希望对大家有所帮助。感兴趣的朋友可以参阅：、等，如有不足之处请留言，小编会及时更正。

感谢朋友们对网站的支持！