MYSQL子查詢和巢狀查詢優化例項解析
阿新 • • 發佈:2020-01-09
查詢遊戲歷史成績最高分前100
Sql程式碼
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程式碼
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;
執行時間僅為0.22秒,比原來的25秒提高了10000倍
查詢當天遊戲最好成績
Sql程式碼
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
像查詢裡:
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') = '$todaytime' )
特別消耗時間
另外,像:
FROM_UNIXTIME(ps1.dateline,'%Y%m%d') = '$todaytime'
這樣的語句會導致索引無效,因為對每個dataline的值都需要用函式計算一遍,需要調整為:
Sql程式碼
AND ps1.dateline >= UNIX_TIMESTAMP('$todaytime')
//更改後
Sql程式碼
SELECT ps. *,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
對於每個球員,找出球員號碼,名字以及他所引起的罰款的號碼,但只是針對那些至少有兩次罰款的球員。
更緊湊的查詢,在FROM子句中放置一個子查詢。
Sql程式碼
SELECT PLAYERNO,NAME,NUMBER FROM (SELECT PLAYERNO,(SELECT COUNT(*) FROM PENALTIES WHERE PENALTIES.PLAYERNO = PLAYERS.PLAYERNO) AS NUMBER FROM PLYERS) AS PN WHERE NUMBER>=2
FROM子句中的子查詢決定了每個球員的號碼,名字和罰款的編號。接下來,這個號碼變成了中間結果中的一列。然後指定了一個條件(NUMBER>=2);最後,獲取SELECT子句中的列。
總結
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