MySQL SELECT 子查询

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 having sum(slary)>5000;
+------+------------+
| xing | sum(slary) |
+------+------------+
| guo  |       9800 |
| hao  |       6400 |
+------+------------+
2 rows in set (0.00 sec)

　　

五、级联

级联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 |
+----+----------+-------+
|  1 | guoding  |  3200 |
|  2 | dingtao  |  2800 |
|  3 | haofugui |  3500 |
|  4 | guoming  |  4000 |
|  5 | haotian  |  2900 |
|  6 | fengfei  |  3200 |
|  7 | guoting  |  2600 |
+----+----------+-------+
7 rows in set (0.00 sec)

mysql> select * from post;
+-----+----------+-------+
| num | name     | level |
+-----+----------+-------+
|   1 |          | 6     |
|   2 | fengfei  | 4     |
|   3 | haotian  | 3     |
|   4 | guoming  | 1     |
|   5 | haofugui | 2     |
|   6 | dingtao  | 5     |
|   7 | guoding  | 4     |
+-----+----------+-------+
7 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  | 4     |  3200 |
| haotian  | 3     |  2900 |
| guoming  | 1     |  4000 |
| haofugui | 2     |  3500 |
| dingtao  | 5     |  2800 |
| guoding  | 4     |  3200 |
+----------+-------+-------+

 

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  | 4     |  3200 |
| dingtao  | 5     |  2800 |
| haofugui | 2     |  3500 |
| guoming  | 1     |  4000 |
| haotian  | 3     |  2900 |
| fengfei  | 4     |  3200 |
| NULL     | NULL  |  2600 |
+----------+-------+-------+
7 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 |
+----------+-------+-------+
|          | 6     |  NULL |
| fengfei  | 4     |  3200 |
| haotian  | 3     |  2900 |
| guoming  | 1     |  4000 |
| haofugui | 2     |  3500 |
| dingtao  | 5     |  2800 |
| guoding  | 4     |  3200 |
+----------+-------+-------+
7 rows in set (0.00 sec)

 

MySQL SELECT 子查询

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