8种最坑SQL语法，工作中踩过吗？

　　sql语句的执行顺序：  FROM   ON     JOIN   WHERE   GROUP BY   HAVING   SELECT  DISTINCT   ORDER BY   LIMIT   1、LIMIT 语句
　　分页查询是最常用的场景之一，但也通常也是最容易出问题的地方。比如对于下面简单的语句，一般 DBA 想到的办法是在 type, name, create_time 字段上加组合索引。这样条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 解决该问题就到此为止。但当 LIMIT 子句变成 ＂LIMIT 1000000,10＂ 时，程序员仍然会抱怨：我只取10条记录为什么还是慢？
　　要知道数据库也并不知道第1000000条记录从什么地方开始，即使有索引也需要从头计算一次。出现这种性能问题，多数情形下是程序员偷懒了。
　　在前端数据浏览翻页，或者大数据分批导出等场景下，是可以将上一页的最大值当成参数作为查询条件的。SQL 重新设计如下：SELECT   * FROM     operation WHERE    type = ＂SQLStats＂ AND      name = ＂SlowLog＂ AND      create_time > ＂2017-03-16 14:00:00＂ ORDER BY create_time limit 10;
　　在新设计下查询时间基本固定，不会随着数据量的增长而发生变化。2、隐式转换
　　SQL语句中查询变量和字段定义类型不匹配是另一个常见的错误。比如下面的语句：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 的定义为 varchar(20)，MySQL 的策略是将字符串转换为数字之后再比较。函数作用于表字段，索引失效。
　　上述情况可能是应用程序框架自动填入的参数，而不是程序员的原意。现在应用框架很多很繁杂，使用方便的同时也小心它可能给自己挖坑。3、关联更新、删除
　　虽然 MySQL5.6 引入了物化特性，但需要特别注意它目前仅仅针对查询语句的优化。对于更新或删除需要手工重写成 JOIN。
　　比如下面 UPDATE 语句，MySQL 实际执行的是循环/嵌套子查询（DEPENDENT SUBQUERY)，其执行时间可想而知。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);
　　执行计划：+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+ | 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                         | +----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+
　　重写为 JOIN 之后，子查询的选择模式从 DEPENDENT SUBQUERY 变成 DERIVED，执行速度大大加快，从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＂
　　执行计划简化为：+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+ | 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 不能利用索引进行混合排序。但在某些场景，还是有机会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
　　执行计划显示为全表扫描：+----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+ | 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1两种状态，我们按照下面的方法重写后，执行时间从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语句
　　MySQL 对待 EXISTS 子句时，仍然采用嵌套子查询的执行方式。如下面的 SQL 语句：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
　　执行计划为：+----+--------------------+-------+------+-----+------------------------------------------+---------+-------+---------+ -----+ | 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 更改为 join，能够避免嵌套子查询，将执行时间从1.93秒降低为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
　　新的执行计划：+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+ | 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、条件下推
　　外部查询条件不能够下推到复杂的视图或子查询的情况有：
　　1、聚合子查询；2、含有 LIMIT 的子查询；3、UNION 或 UNION ALL 子查询；4、输出字段中的子查询；
　　如下面的语句，从执行计划可以看出其条件作用于聚合子查询之后：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     |  | ref   |    |  | 514     | const |    2 | Using where | |  2 | DERIVED     | operation  | index | idx_4         | idx_4       | 519     | NULL  |   20 | Using index | +----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+
　　确定从语义上查询条件可以直接下推后，重写如下：SELECT target,        Count(*) FROM   operation WHERE  target = ＂rm-xxxx＂ GROUP  BY target
　　执行计划变为：+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+ | 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 | +----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
　　关于 MySQL 外部条件不能下推的详细解释说明请参考以前文章：MySQL · 性能优化 · 条件下推到物化表 http://mysql.taobao.org/monthly/2016/07/087、提前缩小范围
　　先上初始 SQL 语句：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
　　该SQL语句原意是：先做一系列的左连接，然后排序取前15条记录。从执行计划也可以看出，最后一步估算排序记录数为90万，时间消耗为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 条件以及排序均针对最左主表，因此可以先对 my_order 排序提前缩小数据量再做左连接。SQL 重写后如下，执行时间缩小为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
　　再检查执行计划：子查询物化后（select_type=DERIVED)参与 JOIN。虽然估算行扫描仍然为90万，但是利用了索引以及 LIMIT 子句后，实际执行时间变得很小。+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+ | id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows   | Extra                                              | +----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+ |  1 | PRIMARY     |  | 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、中间结果集下推
　　再来看下面这个已经初步优化过的例子(左连接中的主表优先作用查询条件)：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
　　那么该语句还存在其它问题吗？不难看出子查询 c 是全表聚合查询，在表数量特别大的情况下会导致整个语句的性能下降。
　　其实对于子查询 c，左连接最后结果集只关心能和主表 resourceid 能匹配的数据。因此我们可以重写语句如下，执行时间从原来的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
　　但是子查询 a 在我们的SQL语句中出现了多次。这种写法不仅存在额外的开销，还使得整个语句显的繁杂。使用 WITH 语句再次重写：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  总结
　　数据库编译器产生执行计划，决定着SQL的实际执行方式。但是编译器只是尽力服务，所有数据库的编译器都不是尽善尽美的。
　　上述提到的多数场景，在其它数据库中也存在性能问题。了解数据库编译器的特性，才能避规其短处，写出高性能的SQL语句。
　　程序员在设计数据模型以及编写SQL语句时，要把算法的思想或意识带进来。
　　编写复杂SQL语句要养成使用 WITH 语句的习惯。简洁且思路清晰的SQL语句也能减小数据库的负担 。