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_TIMESТAMP(‘$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_TIMESТAMP( ‘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

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