Mysql - 开发技巧(二)

本文中的涉及到的表在https://github.com/YangBaohust/my_sql中
本文衔接Mysql - 巧用join来优化sql(https://www.cnblogs.com/ddzj01/p/11346954.html)

1. 行转列

例子：找出取经组中每人打怪的总数，并一行显示
原始数据：

+-----------+-------+
| user_name | kills |
+-----------+-------+
| 孙悟空    |    10 |
| 孙悟空    |     2 |
| 孙悟空    |    12 |
| 孙悟空    |    22 |
| 猪八戒    |    20 |
| 猪八戒    |    17 |
| 猪八戒    |    35 |
| 沙僧      |     3 |
| 沙僧      |     9 |
| 沙僧      |     5 |
+-----------+-------+

想要的数据：

+-----------+-----------+--------+
| 孙悟空    | 猪八戒    | 沙僧   |
+-----------+-----------+--------+
|        46 |        72 |     17 |
+-----------+-----------+--------+

sql如下：
select sum(case when user_name='孙悟空' then kills end) '孙悟空',
sum(case when user_name='猪八戒' then kills end) '猪八戒',
sum(case when user_name='沙僧' then kills end) '沙僧'
from user1_kills;

2. 使用子查询避免重复数据

例子：找出取经组中有打怪记录的人
取经组user1

+----+-----------+-----------------------------+---------------------------------+
| id | user_name | comment                     | mobile                          |
+----+-----------+-----------------------------+---------------------------------+
|  1 | 唐僧      | 旃檀功德佛                  | 138245623,021-382349            |
|  2 | 孙悟空    | 此人在悟空的朋友圈          | 159384292,022-483432,+86-392432 |
|  3 | 猪八戒    | 净坛使者                    | 183208243,055-8234234           |
|  4 | 沙僧      | 金身罗汉                    | 293842295,098-2383429           |
|  5 | NULL      | 白龙马                      | 993267899                       |
+----+-----------+-----------------------------+---------------------------------+

打怪记录user1_kills

+----+-----------+---------------------+-------+
| id | user_name | timestr             | kills |
+----+-----------+---------------------+-------+
|  1 | 孙悟空    | 2013-01-10 00:00:00 |    10 |
|  2 | 孙悟空    | 2013-02-01 00:00:00 |     2 |
|  3 | 孙悟空    | 2013-02-05 00:00:00 |    12 |
|  4 | 孙悟空    | 2013-02-12 00:00:00 |    22 |
|  5 | 猪八戒    | 2013-01-11 00:00:00 |    20 |
|  6 | 猪八戒    | 2013-02-07 00:00:00 |    17 |
|  7 | 猪八戒    | 2013-02-08 00:00:00 |    35 |
|  8 | 沙僧      | 2013-01-10 00:00:00 |     3 |
|  9 | 沙僧      | 2013-01-22 00:00:00 |     9 |
| 10 | 沙僧      | 2013-02-11 00:00:00 |     5 |
+----+-----------+---------------------+-------+

join写法：
select a.id, a.user_name from user1 a join user1_kills b on (a.user_name = b.user_name);

+----+-----------+
| id | user_name |
+----+-----------+
|  2 | 孙悟空    |
|  2 | 孙悟空    |
|  2 | 孙悟空    |
|  2 | 孙悟空    |
|  3 | 猪八戒    |
|  3 | 猪八戒    |
|  3 | 猪八戒    |
|  4 | 沙僧      |
|  4 | 沙僧      |
|  4 | 沙僧      |
+----+-----------+

可以看到通过join写法，会存在一些重复记录，此时需要在sql语句中加distinct，即select distinct a.id...

in写法：
select user_name from user1 where user_name in (select user_name from user1_kills2);

+-----------+
| user_name |
+-----------+
| 孙悟空    |
| 猪八戒    |
| 沙僧      |
+-----------+

因此不同的写法如果存在结果集数量不一致的情况，可以看看是否有重复数据

3. 多列过滤

例子：查询出取经组中每人打怪最多的日期
user1_kills表，取经路上杀的妖怪数量

+----+-----------+---------------------+-------+
| id | user_name | timestr             | kills |
+----+-----------+---------------------+-------+
|  1 | 孙悟空    | 2013-01-10 00:00:00 |    10 |
|  2 | 孙悟空    | 2013-02-01 00:00:00 |     2 |
|  3 | 孙悟空    | 2013-02-05 00:00:00 |    12 |
|  4 | 孙悟空    | 2013-02-12 00:00:00 |    22 |
|  5 | 猪八戒    | 2013-01-11 00:00:00 |    20 |
|  6 | 猪八戒    | 2013-02-07 00:00:00 |    17 |
|  7 | 猪八戒    | 2013-02-08 00:00:00 |    35 |
|  8 | 沙僧      | 2013-01-10 00:00:00 |     3 |
|  9 | 沙僧      | 2013-01-22 00:00:00 |     9 |
| 10 | 沙僧      | 2013-02-11 00:00:00 |     5 |
+----+-----------+---------------------+-------+

在我的另一篇文章https://www.cnblogs.com/ddzj01/p/11346954.html中提到了两种sql写法
聚合子查询写法：
select * from user1_kills a where a.kills = (select max(b.kills) from user1_kills b where b.user_name = a.user_name);
join写法：
select a.* from user1_kills a join user1_kills b on (a.user_name = b.user_name) group by a.id, a.user_name, a.timestr, a.kills having a.kills = max(b.kills);

这里介绍另一种写法：
select * from user1_kills a where (a.user_name, a.kills) in (select b.user_name, max(b.kills) from user1_kills b group by b.user_name);

+----+-----------+---------------------+-------+
| id | user_name | timestr             | kills |
+----+-----------+---------------------+-------+
|  4 | 孙悟空    | 2013-02-12 00:00:00 |    22 |
|  7 | 猪八戒    | 2013-02-08 00:00:00 |    35 |
|  9 | 沙僧      | 2013-01-22 00:00:00 |     9 |
+----+-----------+---------------------+-------+

4. 计算个人所得税

salary表

+----+-----------+-----------------+-------+
| id | user_name | comment         | money |
+----+-----------+-----------------+-------+
|  1 | 唐僧      | 旃檀功德佛      | 35000 |
|  2 | 孙悟空    | 斗战胜佛        | 28000 |
|  3 | 猪八戒    | 净坛使者        | 15000 |
|  4 | 沙僧      | 金身罗汉        |  8000 |
+----+-----------+-----------------+-------+

tax表

+----+-------+---------+------+
| id | low   | high    | rate |
+----+-------+---------+------+
|  1 |     0 |    1500 | 0.03 |
|  2 |  1500 |    4500 | 0.10 |
|  3 |  4500 |    9000 | 0.20 |
|  4 |  9000 |   35000 | 0.25 |
|  5 | 35000 |   55000 | 0.30 |
|  6 | 55000 | 1000000 | 0.35 |
+----+-------+---------+------+

例子：算出取经组中每人的个人所得税

计算个人所得税，属于经典题目了，下面看看用sql如何实现
首先，直接对两表进行笛卡尔连接，为了节约篇幅，只取唐僧的数据查看
select * from salary a join tax b order by user_name, low;

+----+-----------+-----------------+-------+----+-------+---------+------+
| id | user_name | comment         | money | id | low   | high    | rate |
+----+-----------+-----------------+-------+----+-------+---------+------+
|  1 | 唐僧      | 旃檀功德佛      | 35000 |  1 |     0 |    1500 | 0.03 |
|  1 | 唐僧      | 旃檀功德佛      | 35000 |  2 |  1500 |    4500 | 0.10 |
|  1 | 唐僧      | 旃檀功德佛      | 35000 |  3 |  4500 |    9000 | 0.20 |
|  1 | 唐僧      | 旃檀功德佛      | 35000 |  4 |  9000 |   35000 | 0.25 |
|  1 | 唐僧      | 旃檀功德佛      | 35000 |  5 | 35000 |   55000 | 0.30 |
|  1 | 唐僧      | 旃檀功德佛      | 35000 |  6 | 55000 | 1000000 | 0.35 |
+----+-----------+-----------------+-------+----+-------+---------+------+

可以看到最后两条数据不需要，因为它的low都大于等于money，所以可以加上连接条件a.money > b.low，排除不需要的行
select * from salary a join tax b on (a.money > b.low) order by user_name, low;

+----+-----------+-----------------+-------+----+------+-------+------+
| id | user_name | comment         | money | id | low  | high  | rate |
+----+-----------+-----------------+-------+----+------+-------+------+
|  1 | 唐僧      | 旃檀功德佛      | 35000 |  1 |    0 |  1500 | 0.03 |
|  1 | 唐僧      | 旃檀功德佛      | 35000 |  2 | 1500 |  4500 | 0.10 |
|  1 | 唐僧      | 旃檀功德佛      | 35000 |  3 | 4500 |  9000 | 0.20 |
|  1 | 唐僧      | 旃檀功德佛      | 35000 |  4 | 9000 | 35000 | 0.25 |
+----+-----------+-----------------+-------+----+------+-------+------+

接下来只需要计算每一行的税费即可，计算的标准则为least(money, high) - low，因此sql如下
select a.user_name, sum((least(a.money, b.high) - b.low) * b.rate) total from salary a join tax b on (a.money > b.low) group by a.user_name;

+-----------+---------+
| user_name | total   |
+-----------+---------+
| 唐僧      | 7745.00 |
| 孙悟空    | 5995.00 |
| 沙僧      | 1045.00 |
| 猪八戒    | 2745.00 |
+-----------+---------+

不管是开发人员还是dba，都应该熟练掌握sql技巧，本文例子取自于慕课网《sql开发技巧》。如果文章对你有帮助，不妨点个赞，支持下博主。

原文地址：https://www.cnblogs.com/ddzj01/p/11351622.html