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 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)
