MySQL查询命令_SELECT 子查询

首先创建一个table

mysql> create table Total (id int AUTO_INCREMENT PRIMARY KEY,name char(20),stu_num int NOT NULL,teacher_num int NOT NULL);
Query OK, 0 rows affected (0.11 sec)

mysql> insert into Total (name,stu_num,teacher_num) values ("小学1",400,20),("小学2",356,24),("小学3",403,19),("小学4",367,26),("小学5",373,20),("小学6",406,21);
Query OK, 6 rows affected (0.00 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> select * from Total;
+----+---------+---------+-------------+
| id | name    | stu_num | teacher_num |
+----+---------+---------+-------------+
|  1 | 小学1 |     400 |          20 |
|  2 | 小学2 |     356 |          24 |
|  3 | 小学3 |     403 |          19 |
|  4 | 小学4 |     367 |          26 |
|  5 | 小学5 |     373 |          20 |
|  6 | 小学6 |     406 |          21 |
+----+---------+---------+-------------+
6 rows in set (0.00 sec)

接下来就以这个表格为例，介绍select语句的深层运用。

一、查询不重复的记录

语句：select distinct [属性1,属性2] from 表名 

说明：distinct必须放在最前的位置；

          distinct只能使用需要去重的字段进行操作，也就是说如果distinct采用了name，stu_num两个字段，但是后面想利用id进行排序是不可以的，因为只能对name和stu_name两个字段进行操作；

          distinct去重多个字段时，含义是几个字段同时重复时才会被过滤。

示例：

mysql> select distinct name,teacher_num from Total;
+---------+-------------+
| name    | teacher_num |
+---------+-------------+
| 小学1 |          20 |
| 小学2 |          24 |
| 小学3 |          19 |
| 小学4 |          26 |
| 小学5 |          20 |
| 小学6 |          21 |
+---------+-------------+
6 rows in set (0.00 sec)

mysql> select distinct teacher_num from Total;
+-------------+
| teacher_num |
+-------------+
|          20 |
|          24 |
|          19 |
|          26 |
|          21 |
+-------------+
5 rows in set (0.00 sec)

二、排序

语法：select * from 表名 [where 条件] [ order by 属性1 [desc/asc],属性2 [desc/asc]... ];

说明：desc 降序排列，asc 升序排列；

             order by 可以有多个参数，每个排序参数可以有不同的排序顺序；

             如果第一个排序字段的值一样，则按照第二个排序字段进行排序；

             如果只有一个排序字段，则字段值相同的记录将会无序排列。

示例：

mysql> select id,name from Total where id<4 order by teacher_num desc;
+----+---------+
| id | name    |
+----+---------+
|  2 | 小学2 |
|  1 | 小学1 |
|  3 | 小学3 |
+----+---------+
3 rows in set (0.00 sec)

三、限制

语句：select ... [limit 起始偏移量,行数];    或    select ... [limit 行数] offset 偏移量;

说明：.默认情况下，起始偏移量为0

示例：

mysql> select * from Total order by teacher_num asc;
+----+---------+---------+-------------+
| id | name    | stu_num | teacher_num |
+----+---------+---------+-------------+
|  3 | 小学3 |     403 |          19 |
|  1 | 小学1 |     400 |          20 |
|  5 | 小学5 |     373 |          20 |
|  6 | 小学6 |     406 |          21 |
|  2 | 小学2 |     356 |          24 |
|  4 | 小学4 |     367 |          26 |
+----+---------+---------+-------------+
6 rows in set (0.00 sec)

mysql> select * from Total limit 4 offset 1;
+----+---------+---------+-------------+
| id | name    | stu_num | teacher_num |
+----+---------+---------+-------------+
|  2 | 小学2 |     356 |          24 |
|  3 | 小学3 |     403 |          19 |
|  4 | 小学4 |     367 |          26 |
|  5 | 小学5 |     373 |          20 |
+----+---------+---------+-------------+
4 rows in set (0.00 sec)
mysql> select * from Total order by teacher_num asc limit 4 offset 1;
+----+---------+---------+-------------+
| id | name    | stu_num | teacher_num |
+----+---------+---------+-------------+
|  1 | 小学1 |     400 |          20 |
|  5 | 小学5 |     373 |          20 |
|  6 | 小学6 |     406 |          21 |
|  2 | 小学2 |     356 |          24 |
+----+---------+---------+-------------+
4 rows in set (0.00 sec)

　mysql> (select * from Total limit 4 offset 1) order by teacher_num asc;
　+----+---------+---------+-------------+
　| id | name | stu_num | teacher_num |
　+----+---------+---------+-------------+
　| 3 | 小学3 | 403 | 19 |
　| 5 | 小学5 | 373 | 20 |
　| 2 | 小学2 | 356 | 24 |
　| 4 | 小学4 | 367 | 26 |
　+----+---------+---------+-------------+
　4 rows in set (0.00 sec)

四、聚合

语句：select 字段 fun_name from 表名 [where 条件] [group by 属性1,属性2...] [with rollup] [having 条件];

说明：fun_name表示要做的聚合操作，也就是聚合函数，常用的有sum（求和）、count(*)（记录数）、max（最大值）、min（最小值）等；

            group by 表示要进行分类聚合的字段，比如要按照部门分类统计员工数量；

            with rollup是可选项，表示是否对分类聚合后的结果进行再汇总；

            having 表示对分类后的结果再进行条件过滤。

示例：

mysql> create table Staff
    -> (id int AUTO_INCREMENT PRIMARY KEY,
    -> xing char(20),
    -> ming char(20),
    -> slary int NOT NULL);
Query OK, 0 rows affected (0.07 sec)

mysql> insert into Staff (xing,ming,slary) value('guo','ding',3200),
    -> ('ding','tao',2800),
    -> ('hao','fugui',3500),
    -> ('guo','ming',4000),
    -> ('hao','tian',2900),
    -> ('feng','fei','3200'),
    -> ('guo','ting',2600);
Query OK, 7 rows affected (0.00 sec)
Records: 7  Duplicates: 0  Warnings: 0

mysql> select * from Staff;
+----+------+-------+-------+
| id | xing | ming  | slary |
+----+------+-------+-------+
|  8 | guo  | ding  |  3200 |
|  9 | ding | tao   |  2800 |
| 10 | hao  | fugui |  3500 |
| 11 | guo  | ming  |  4000 |
| 12 | hao  | tian  |  2900 |
| 13 | feng | fei   |  3200 |
| 14 | guo  | ting  |  2600 |
+----+------+-------+-------+
7 rows in set (0.00 sec)

mysql> select count('xing') as xing_num from Staff where xing='guo';   //此处涉及了新的语法
+----------+
| xing_num |
+----------+
|        3 |
+----------+
1 row in set (0.00 sec)

mysql> select sum(slary) from Staff;
+------------+
| sum(slary) |
+------------+
|      22200 |
+------------+
1 row in set (0.00 sec)

mysql> select xing,sum(slary) from Staff group by xing;
+------+------------+
| xing | sum(slary) |
+------+------------+
| ding |       2800 |
| feng |       3200 |
| guo  |       9800 |
| hao  |       6400 |
+------+------------+
4 rows in set (0.00 sec)

mysql> select xing,sum(slary) from Staff group by xing with rollup;
+------+------------+
| xing | sum(slary) |
+------+------------+
| ding |       2800 |
| feng |       3200 |
| guo  |       9800 |
| hao  |       6400 |
| NULL |      22200 |
+------+------------+
5 rows in set (0.00 sec)

mysql> select xing,sum(slary) from Staff group by xing having sum(slary)>5000;
+------+------------+
| xing | sum(slary) |
+------+------------+
| guo  |       9800 |
| hao  |       6400 |
+------+------------+
2 rows in set (0.00 sec)

　　

五、表连接

表连接分为内连接和外连接，其中内连接仅选出两张表中互相匹配的记录，外连接会选出其他不匹配的记录。

假设有以下Staff表和post表：

mysql> select * from staff;
+----+----------+-------+
| id | name     | slary |
+----+----------+-------+
|   | guoding  |   |
|   | dingtao  |   |
|   | haofugui |   |
|   | guoming  |   |
|   | haotian  |   |
|   | fengfei  |   |
|   | guoting  |   |
+----+----------+-------+
 rows in set (0.00 sec)

mysql> select * from post;
+-----+----------+-------+
| num | name     | level |
+-----+----------+-------+
|    |          |      |
|    | fengfei  |      |
|    | haotian  |      |
|    | guoming  |      |
|    | haofugui |      |
|    | dingtao  |      |
|    | guoding  |      |
+-----+----------+-------+
 rows in set (0.00 sec)

示例：（内连接），语句：select 属性1,属性2 from 表1,表2 where 表1.属性值=表2.属性值

mysql> select post.name,level,slary from staff,post where staff.name=post.name;
+----------+-------+-------+
| name     | level | slary |
+----------+-------+-------+
| fengfei  |      |   |
| haotian  |      |   |
| guoming  |      |   |
| haofugui |      |   |
| dingtao  |      |   |
| guoding  |      |   |
+----------+-------+-------+

外连接又分为左连接与右连接：

左连接：包含所有左边表中的记录，甚至是右边表中没有和他匹配的记录。

右连接：包含所有右边表中的记录，甚至是右边表中没有和他匹配的记录。

示例：（左连接），语句：select 属性1,属性2 from 表1 left join 表2 on 表1.属性值=表2.属性值

mysql> select post.name,level,slary from staff left join post on staff.name=post.name;
+----------+-------+-------+
| name     | level | slary |
+----------+-------+-------+
| guoding  |      |   |
| dingtao  |      |   |
| haofugui |      |   |
| guoming  |      |   |
| haotian  |      |   |
| fengfei  |      |   |
| NULL     | NULL  |   |
+----------+-------+-------+
 rows in set (0.00 sec)

示例：（右连接），语句：select 属性1,属性2 from 表1 right join 表2 on 表1.属性值=表2.属性值

mysql> select post.name,level,slary from staff right join post on staff.name=post.name;
+----------+-------+-------+
| name     | level | slary |
+----------+-------+-------+
|          |      |  NULL |
| fengfei  |      |   |
| haotian  |      |   |
| guoming  |      |   |
| haofugui |      |   |
| dingtao  |      |   |
| guoding  |      |   |
+----------+-------+-------+
 rows in set (0.00 sec)

需要说明的是，如果根据ON给出的连接条件，两个欲进行连接查询的表中存在一对多的匹配关系，则会输出对应的多条记录。

比如有以下两个表：

mysql> select * from YSHA;
+------+------+
| code | NAME |
+------+------+
|     | A1   |
|     | A2   |
+------+------+
 rows in set (0.01 sec)

mysql> select * from YSHB;
+------+------+
| code | col  |
+------+------+
|     | Row1 |
|     | Row2 |
|     | Row3 |
|     | Row1 |
+------+------+
 rows in set (0.00 sec)

接下来以YSHA.code=YSHB.code作为连接条件：

mysql> select * from YSHA left join YSHB on YSHA.code=YSHB.code;
+------+------+------+------+
| code | NAME | code | col  |
+------+------+------+------+
|     | A1   |     | Row1 |
|     | A1   |     | Row2 |
|     | A1   |     | Row3 |
|     | A2   | NULL | NULL |
+------+------+------+------+

mysql> select * from YSHA right join YSHB on YSHA.code=YSHB.code;
+------+------+------+------+
| code | NAME | code | col  |
+------+------+------+------+
|     | A1   |     | Row1 |
|     | A1   |     | Row2 |
|     | A1   |     | Row3 |
| NULL | NULL |     | Row1 |
+------+------+------+------+

六、联合查询

MySQL使用UNION和UNION ALL实现数据的联合查询。

假设有以下4个table：

mysql> select * from staff;
+----+----------+-------+
| id | name     | slary |
+----+----------+-------+
|   | guoding  |   |
|   | dingtao  |   |
|   | haofugui |   |
|   | guoming  |   |
|   | haotian  |   |
|   | fengfei  |   |
|   | guoting  |   |
+----+----------+-------+
rows in set (0.00 sec)

mysql> select * from staff_1;
+----+----------+-------+
| id | name     | slary |
+----+----------+-------+
|   | guoding  |   |
|   | liding   |   |
|   | haofugui |   |
|   | xiaoli   |   |
|   | yazhi    |   |
|   | yuanfei  |   |
|   | guoting  |   |
+----+----------+-------+
rows in set (0.00 sec)

mysql> select * from staff_2;
+----+-------+----------+
| id | slary | name     |
+----+-------+----------+
|   |   | guoding  |
|   |   | liding   |
|   |   | haofugui |
|   |   | xiaoli   |
|   |   | yazhi    |
|   |   | yuanfei  |
|   |   | guoting  |
+----+-------+----------+
rows in set (0.00 sec)

mysql> select * from staff_3;
+----------+-------+
| name     | slary |
+----------+-------+
| guoding  |   |
| liding   |   |
| haofugui |   |
| xiaoli   |   |
+----------+-------+
rows in set (0.00 sec)

1. UNION

语句：SELECT column_name FROM table1 UNION SELECT column_name FROM table2

说明：1）用于合并两个或多个 SELECT 语句的结果集，并消去表中任何重复行；
           2）UNION 内部的 SELECT 语句必须拥有相同数量的列，列也必须拥有相似的数据类型；
           3）同时，每条 SELECT 语句中的列的顺序必须相同；

           4）如果子句中有order by,limit等，需用括号()包起来，推荐放到所有子句之后，即对最终合并的结果来排序或筛选。

mysql> select * from staff union select * from staff_1;
+----+----------+-------+
| id | name     | slary |
+----+----------+-------+
|   | guoding  |   |
|   | dingtao  |   |
|   | haofugui |   |
|   | guoming  |   |
|   | haotian  |   |
|   | fengfei  |   |
|   | guoting  |   |
|   | liding   |   |
|   | xiaoli   |   |
|   | yazhi    |   |
|   | yuanfei  |   |
|   | guoting  |   |
+----+----------+-------+
rows in set (0.00 sec)

mysql> select * from staff union select * from staff_2;
+----+----------------------+----------------------+
| id | name                 | slary                |
+----+----------------------+----------------------+
|   | guoding              |                  |
|   | dingtao              |                  |
|   | haofugui             |                  |
|   | guoming              |                  |
|   | haotian              |                  |
|   | fengfei              |                  |
|   | guoting              |                  |
|   |                  | guoding              |
|   |                  | liding               |
|   |                  | haofugui             |
|   |                  | xiaoli               |
|   |                  | yazhi                |
|   |                  | yuanfei              |
|   |                  | guoting              |
+----+----------------------+----------------------+
rows in set (0.00 sec)

mysql> select * from staff union select name from staff_1;
ERROR  (): The used SELECT statements have a different number of columns

mysql> select * from staff union select * from staff_3;
ERROR  (): The used SELECT statements have a different number of columns

mysql> (select * from staff order by id asc) union (select * from staff_1 order by slary desc);
+----+----------+-------+
| id | name     | slary |
+----+----------+-------+
|   | guoding  |   |
|   | dingtao  |   |
|   | haofugui |   |
|   | guoming  |   |
|   | haotian  |   |
|   | fengfei  |   |
|   | guoting  |   |
|   | liding   |   |
|   | xiaoli   |   |
|   | yazhi    |   |
|   | yuanfei  |   |
|   | guoting  |   |
+----+----------+-------+
rows in set (0.00 sec)

2. UNION ALL

作用及规则与UNION相同，区别是，UNION ALL不消除重复行

mysql> select * from staff union select * from staff_1;
+----+----------+-------+
| id | name     | slary |
+----+----------+-------+
|   | guoding  |   |
|   | dingtao  |   |
|   | haofugui |   |
|   | guoming  |   |
|   | haotian  |   |
|   | fengfei  |   |
|   | guoting  |   |
|   | liding   |   |
|   | xiaoli   |   |
|   | yazhi    |   |
|   | yuanfei  |   |
|   | guoting  |   |
+----+----------+-------+
rows in set (0.00 sec)

mysql> select * from staff union all select * from staff_1;
+----+----------+-------+
| id | name     | slary |
+----+----------+-------+
|   | guoding  |   |
|   | dingtao  |   |
|   | haofugui |   |
|   | guoming  |   |
|   | haotian  |   |
|   | fengfei  |   |
|   | guoting  |   |
|   | guoding  |   |
|   | liding   |   |
|   | haofugui |   |
|   | xiaoli   |   |
|   | yazhi    |   |
|   | yuanfei  |   |
|   | guoting  |   |
+----+----------+-------+
rows in set (0.00 sec)