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本篇文章為大家展示了可能寫錯的 SQL 用法有哪些，內(nèi)容簡明扼要并且容易理解，絕對能使你眼前一亮，通過這篇文章的詳細(xì)介紹希望你能有所收獲。
1、LIMIT 語句
分頁查詢是最常用的場景之一，但也通常也是最容易出問題的地方。比如對于下面簡單的語句，一般 DBA 想到的辦法是在 type, name, create_time 字段上加組合索引。這樣條件排序都能有效的利用到索引，性能迅速提升。
SELECT * FROM operation WHERE type = SQLStats AND name = SlowLog ORDER BY create_time LIMIT 1000, 10;好吧，可能 90% 以上的 DBA 解決該問題就到此為止。但當(dāng) LIMIT 子句變成“LIMIT 1000000,10”時，程序員仍然會抱怨：我只取 10 條記錄為什么還是慢？
要知道數(shù)據(jù)庫也并不知道第 1000000 條記錄從什么地方開始，即使有索引也需要從頭計(jì)算一次。出現(xiàn)這種性能問題，多數(shù)情形下是程序員偷懶了。
在前端數(shù)據(jù)瀏覽翻頁，或者大數(shù)據(jù)分批導(dǎo)出等場景下，是可以將上一頁的最大值當(dāng)成參數(shù)作為查詢條件的。SQL 重新設(shè)計(jì)如下：
SELECT * FROM operation WHERE type = SQLStats AND name = SlowLog AND create_time 2017-03-16 14:00:00 ORDER BY create_time limit 10;在新設(shè)計(jì)下查詢時間基本固定，不會隨著數(shù)據(jù)量的增長而發(fā)生變化。
2、隱式轉(zhuǎn)換
SQL 語句中查詢變量和字段定義類型不匹配是另一個常見的錯誤。比如下面的語句：
mysql explain extended SELECT * FROM my_balance b WHERE b.bpn = 14000000123 AND b.isverified IS NULL ; mysql show warnings; | Warning | 1739 | Cannot use ref access on index bpn due to type or collation conversion on field bpn其中字段 bpn 的定義為 varchar(20)，MySQL 的策略是將字符串轉(zhuǎn)換為數(shù)字之后再比較。函數(shù)作用于表字段，索引失效。
上述情況可能是應(yīng)用程序框架自動填入的參數(shù)，而不是程序員的原意?，F(xiàn)在應(yīng)用框架很多很繁雜，使用方便的同時也小心它可能給自己挖坑。
3、關(guān)聯(lián)更新、刪除
雖然 MySQL5.6 引入了物化特性，但需要特別注意它目前僅僅針對查詢語句的優(yōu)化。對于更新或刪除需要手工重寫成 JOIN。
比如下面 UPDATE 語句，MySQL 實(shí)際執(zhí)行的是循環(huán) / 嵌套子查詢（DEPENDENT SUBQUERY)，其執(zhí)行時間可想而知。
UPDATE operation o SET status = applying WHERE o.id IN (SELECT id FROM (SELECT o.id, o.status FROM operation o WHERE o.group = 123 AND o.status NOT IN ( done ) ORDER BY o.parent, o.id LIMIT 1) t);執(zhí)行計(jì)劃：
+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+ | 1 | PRIMARY | o | index | | PRIMARY | 8 | | 24 | Using where; Using temporary | | 2 | DEPENDENT SUBQUERY | | | | | | | | Impossible WHERE noticed after reading const tables | | 3 | DERIVED | o | ref | idx_2,idx_5 | idx_5 | 8 | const | 1 | Using where; Using filesort | +----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+重寫為 JOIN 之后，子查詢的選擇模式從 DEPENDENT SUBQUERY 變成 DERIVED，執(zhí)行速度大大加快，從 7 秒降低到 2 毫秒。
UPDATE operation o JOIN (SELECT o.id, o.status FROM operation o WHERE o.group = 123 AND o.status NOT IN ( done ) ORDER BY o.parent, o.id LIMIT 1) t ON o.id = t.id SET status = applying執(zhí)行計(jì)劃簡化為：
+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+ | 1 | PRIMARY | | | | | | | | Impossible WHERE noticed after reading const tables | | 2 | DERIVED | o | ref | idx_2,idx_5 | idx_5 | 8 | const | 1 | Using where; Using filesort | +----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+4、混合排序
MySQL 不能利用索引進(jìn)行混合排序。但在某些場景，還是有機(jī)會使用特殊方法提升性能的。
SELECT * FROM my_order o INNER JOIN my_appraise a ON a.orderid = o.id ORDER BY a.is_reply ASC, a.appraise_time DESC LIMIT 0, 20執(zhí)行計(jì)劃顯示為全表掃描：
+----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra +----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+ | 1 | SIMPLE | a | ALL | idx_orderid | NULL | NULL | NULL | 1967647 | Using filesort | | 1 | SIMPLE | o | eq_ref | PRIMARY | PRIMARY | 122 | a.orderid | 1 | NULL | +----+-------------+-------+--------+---------+---------+---------+-----------------+---------+-+由于 is_reply 只有 0 和 1 兩種狀態(tài)，我們按照下面的方法重寫后，執(zhí)行時間從 1.58 秒降低到 2 毫秒。
SELECT * FROM ((SELECT * FROM my_order o INNER JOIN my_appraise a ON a.orderid = o.id AND is_reply = 0 ORDER BY appraise_time DESC LIMIT 0, 20) UNION ALL (SELECT * FROM my_order o INNER JOIN my_appraise a ON a.orderid = o.id AND is_reply = 1 ORDER BY appraise_time DESC LIMIT 0, 20)) t ORDER BY is_reply ASC, appraisetime DESC LIMIT 20;5、EXISTS 語句
MySQL 對待 EXISTS 子句時，仍然采用嵌套子查詢的執(zhí)行方式。如下面的 SQL 語句：
SELECT * FROM my_neighbor n LEFT JOIN my_neighbor_apply sra ON n.id = sra.neighbor_id AND sra.user_id = xxx WHERE n.topic_status 4 AND EXISTS(SELECT 1 FROM message_info m WHERE n.id = m.neighbor_id AND m.inuser = xxx) AND n.topic_type 5執(zhí)行計(jì)劃為：
+----+--------------------+-------+------+-----+------------------------------------------+---------+-------+---------+ -----+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+--------------------+-------+------+ -----+------------------------------------------+---------+-------+---------+ -----+ | 1 | PRIMARY | n | ALL | | NULL | NULL | NULL | 1086041 | Using where | | 1 | PRIMARY | sra | ref | | idx_user_id | 123 | const | 1 | Using where | | 2 | DEPENDENT SUBQUERY | m | ref | | idx_message_info | 122 | const | 1 | Using index condition; Using where | +----+--------------------+-------+------+ -----+------------------------------------------+---------+-------+---------+ -----+去掉 exists 更改為 join，能夠避免嵌套子查詢，將執(zhí)行時間從 1.93 秒降低為 1 毫秒。
SELECT * FROM my_neighbor n INNER JOIN message_info m ON n.id = m.neighbor_id AND m.inuser = xxx LEFT JOIN my_neighbor_apply sra ON n.id = sra.neighbor_id AND sra.user_id = xxx WHERE n.topic_status 4 AND n.topic_type 5新的執(zhí)行計(jì)劃：
+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+ | 1 | SIMPLE | m | ref | | idx_message_info | 122 | const | 1 | Using index condition | | 1 | SIMPLE | n | eq_ref | | PRIMARY | 122 | ighbor_id | 1 | Using where | | 1 | SIMPLE | sra | ref | | idx_user_id | 123 | const | 1 | Using where | +----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+6、條件下推
外部查詢條件不能夠下推到復(fù)雜的視圖或子查詢的情況有：
聚合子查詢；
含有 LIMIT 的子查詢；
UNION 或 UNION ALL 子查詢；
輸出字段中的子查詢；
如下面的語句，從執(zhí)行計(jì)劃可以看出其條件作用于聚合子查詢之后：
SELECT * FROM (SELECT target, Count(*) FROM operation GROUP BY target) t WHERE target = rm-xxxx+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+ | 1 | PRIMARY | derived2 | ref | auto_key0 | auto_key0 | 514 | const | 2 | Using where | | 2 | DERIVED | operation | index | idx_4 | idx_4 | 519 | NULL | 20 | Using index | +----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+確定從語義上查詢條件可以直接下推后，重寫如下：
SELECT target, Count(*) FROM operation WHERE target = rm-xxxx GROUP BY target執(zhí)行計(jì)劃變?yōu)椋?/p>
+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+ | 1 | SIMPLE | operation | ref | idx_4 | idx_4 | 514 | const | 1 | Using where; Using index | +----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+關(guān)于 MySQL 外部條件不能下推的詳細(xì)解釋說明請參考文章：
http://mysql.taobao.org/monthly/2016/07/087、提前縮小范圍
先上初始 SQL 語句：
SELECT * FROM my_order o LEFT JOIN my_userinfo u ON o.uid = u.uid LEFT JOIN my_productinfo p ON o.pid = p.pid WHERE ( o.display = 0 ) AND ( o.ostaus = 1 ) ORDER BY o.selltime DESC LIMIT 0, 15該 SQL 語句原意是：先做一系列的左連接，然后排序取前 15 條記錄。從執(zhí)行計(jì)劃也可以看出，最后一步估算排序記錄數(shù)為 90 萬，時間消耗為 12 秒。
+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+ | 1 | SIMPLE | o | ALL | NULL | NULL | NULL | NULL | 909119 | Using where; Using temporary; Using filesort | | 1 | SIMPLE | u | eq_ref | PRIMARY | PRIMARY | 4 | o.uid | 1 | NULL | | 1 | SIMPLE | p | ALL | PRIMARY | NULL | NULL | NULL | 6 | Using where; Using join buffer (Block Nested Loop) | +----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+由于最后 WHERE 條件以及排序均針對最左主表，因此可以先對 my_order 排序提前縮小數(shù)據(jù)量再做左連接。SQL 重寫后如下，執(zhí)行時間縮小為 1 毫秒左右。
SELECT * FROM ( SELECT * FROM my_order o WHERE ( o.display = 0 ) AND ( o.ostaus = 1 ) ORDER BY o.selltime DESC LIMIT 0, 15 ) o LEFT JOIN my_userinfo u ON o.uid = u.uid LEFT JOIN my_productinfo p ON o.pid = p.pid ORDER BY o.selltime DESC limit 0, 15再檢查執(zhí)行計(jì)劃：子查詢物化后（select_type=DERIVED) 參與 JOIN。雖然估算行掃描仍然為 90 萬，但是利用了索引以及 LIMIT 子句后，實(shí)際執(zhí)行時間變得很小。
+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+ | 1 | PRIMARY | derived2 | ALL | NULL | NULL | NULL | NULL | 15 | Using temporary; Using filesort | | 1 | PRIMARY | u | eq_ref | PRIMARY | PRIMARY | 4 | o.uid | 1 | NULL | | 1 | PRIMARY | p | ALL | PRIMARY | NULL | NULL | NULL | 6 | Using where; Using join buffer (Block Nested Loop) | | 2 | DERIVED | o | index | NULL | idx_1 | 5 | NULL | 909112 | Using where | +----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+8、中間結(jié)果集下推
再來看下面這個已經(jīng)初步優(yōu)化過的例子 (左連接中的主表優(yōu)先作用查詢條件)：
SELECT a.*, c.allocated FROM ( SELECT resourceid FROM my_distribute d WHERE isdelete = 0 AND cusmanagercode = 1234567 ORDER BY salecode limit 20) a LEFT JOIN ( SELECT resourcesid， sum(ifnull(allocation, 0) * 12345) allocated FROM my_resources GROUP BY resourcesid) c ON a.resourceid = c.resourcesid那么該語句還存在其它問題嗎？不難看出子查詢 c 是全表聚合查詢，在表數(shù)量特別大的情況下會導(dǎo)致整個語句的性能下降。
其實(shí)對于子查詢 c，左連接最后結(jié)果集只關(guān)心能和主表 resourceid 能匹配的數(shù)據(jù)。因此我們可以重寫語句如下，執(zhí)行時間從原來的 2 秒下降到 2 毫秒。
SELECT a.*, c.allocated FROM ( SELECT resourceid FROM my_distribute d WHERE isdelete = 0 AND cusmanagercode = 1234567 ORDER BY salecode limit 20) a LEFT JOIN ( SELECT resourcesid， sum(ifnull(allocation, 0) * 12345) allocated FROM my_resources r, ( SELECT resourceid FROM my_distribute d WHERE isdelete = 0 AND cusmanagercode = 1234567 ORDER BY salecode limit 20) a WHERE r.resourcesid = a.resourcesid GROUP BY resourcesid) c ON a.resourceid = c.resourcesid但是子查詢 a 在我們的 SQL 語句中出現(xiàn)了多次。這種寫法不僅存在額外的開銷，還使得整個語句顯的繁雜。使用 WITH 語句再次重寫：
WITH a AS ( SELECT resourceid FROM my_distribute d WHERE isdelete = 0 AND cusmanagercode = 1234567 ORDER BY salecode limit 20) SELECT a.*, c.allocated FROM a LEFT JOIN ( SELECT resourcesid， sum(ifnull(allocation, 0) * 12345) allocated FROM my_resources r, a WHERE r.resourcesid = a.resourcesid GROUP BY resourcesid) c ON a.resourceid = c.resourcesid數(shù)據(jù)庫編譯器產(chǎn)生執(zhí)行計(jì)劃，決定著 SQL 的實(shí)際執(zhí)行方式。但是編譯器只是盡力服務(wù)，所有數(shù)據(jù)庫的編譯器都不是盡善盡美的。
上述提到的多數(shù)場景，在其它數(shù)據(jù)庫中也存在性能問題。了解數(shù)據(jù)庫編譯器的特性，才能避規(guī)其短處，寫出高性能的 SQL 語句。
程序員在設(shè)計(jì)數(shù)據(jù)模型以及編寫 SQL 語句時，要把算法的思想或意識帶進(jìn)來。
編寫復(fù)雜 SQL 語句要養(yǎng)成使用 WITH 語句的習(xí)慣。簡潔且思路清晰的 SQL 語句也能減小數(shù)據(jù)庫的負(fù)擔(dān)。
上述內(nèi)容就是可能寫錯的 SQL 用法有哪些，你們學(xué)到知識或技能了嗎？如果還想學(xué)到更多技能或者豐富自己的知識儲備，歡迎關(guān)注丸趣 TV 行業(yè)資訊頻道。
