MySQL 03

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目录

单表操作

分组

  • 分组: 将记录按照某个相同的字段进行归类
  • 聚合函数
    • count()
    • sum()
    • max()
    • min()
    • avg()

group by

  • select 聚合函数 [as 列名] from 表名 group by 列名;
mysql> select * from employee;
+----+---------+--------+-----+--------+-----------+
| id | name    | gender | age | salary | depart_id |
+----+---------+--------+-----+--------+-----------+
|  1 | alpha   | male   |  35 |  12000 |         1 |
|  2 | bravo   | male   |  28 |  10000 |         1 |
|  3 | charlie | female |  22 |   7000 |         1 |
|  4 | delta   | female |  37 |  14000 |         1 |
|  5 | echo    | male   |  24 |   8000 |         2 |
|  6 | foxtrot | female |  42 |  20000 |         2 |
|  7 | golf    | female |  29 |  11000 |         2 |
|  8 | hotel   | male   |  31 |  13000 |         2 |
|  9 | india   | male   |  40 |  18000 |         3 |
| 10 | juliet  | male   |  21 |   6000 |         3 |
+----+---------+--------+-----+--------+-----------+
10 rows in set (0.00 sec)

mysql> select gender, count(id) as total from employee group by gender;
+--------+-------+
| gender | total |
+--------+-------+
| female |     4 |
| male   |     6 |
+--------+-------+
2 rows in set (0.00 sec)

mysql> select depart_id, max(salary) from employee group by depart_id;
+-----------+-------------+
| depart_id | max(salary) |
+-----------+-------------+
|         1 |       14000 |
|         2 |       20000 |
|         3 |       18000 |
+-----------+-------------+
3 rows in set (0.00 sec)

mysql> select depart_id, avg(salary) from employee group by depart_id;
+-----------+-------------+
| depart_id | avg(salary) |
+-----------+-------------+
|         1 |       10750 |
|         2 |       13000 |
|         3 |       12000 |
+-----------+-------------+
3 rows in set (0.00 sec)

having

  • having是对group by后的数据进行二次筛选
  • select 聚合函数 [as 列名] from 表名 group by 列名 having 条件;
mysql> select depart_id, avg(salary) from employee group by depart_id;
+-----------+-------------+
| depart_id | avg(salary) |
+-----------+-------------+
|         1 |       10750 |
|         2 |       13000 |
|         3 |       12000 |
+-----------+-------------+
3 rows in set (0.00 sec)

mysql> select depart_id, avg(salary) from employee group by depart_id having depart_id=1;
+-----------+-------------+
| depart_id | avg(salary) |
+-----------+-------------+
|         1 |       10750 |
+-----------+-------------+
1 row in set (0.00 sec)

order by

  • order by 列名 asc 升序
  • order by 列名 desc 降序

mysql> select * from employee order by age asc;
+----+---------+--------+-----+--------+-----------+
| id | name    | gender | age | salary | depart_id |
+----+---------+--------+-----+--------+-----------+
| 10 | juliet  | male   |  21 |   6000 |         3 |
|  3 | charlie | female |  22 |   7000 |         1 |
|  5 | echo    | male   |  24 |   8000 |         2 |
|  2 | bravo   | male   |  28 |  10000 |         1 |
|  7 | golf    | female |  29 |  11000 |         2 |
|  8 | hotel   | male   |  31 |  13000 |         2 |
|  1 | alpha   | male   |  35 |  12000 |         1 |
|  4 | delta   | female |  37 |  14000 |         1 |
|  9 | india   | male   |  40 |  18000 |         3 |
|  6 | foxtrot | female |  42 |  20000 |         2 |
+----+---------+--------+-----+--------+-----------+
10 rows in set (0.00 sec)

mysql> select * from employee order by salary desc;
+----+---------+--------+-----+--------+-----------+
| id | name    | gender | age | salary | depart_id |
+----+---------+--------+-----+--------+-----------+
|  6 | foxtrot | female |  42 |  20000 |         2 |
|  9 | india   | male   |  40 |  18000 |         3 |
|  4 | delta   | female |  37 |  14000 |         1 |
|  8 | hotel   | male   |  31 |  13000 |         2 |
|  1 | alpha   | male   |  35 |  12000 |         1 |
|  7 | golf    | female |  29 |  11000 |         2 |
|  2 | bravo   | male   |  28 |  10000 |         1 |
|  5 | echo    | male   |  24 |   8000 |         2 |
|  3 | charlie | female |  22 |   7000 |         1 |
| 10 | juliet  | male   |  21 |   6000 |         3 |
+----+---------+--------+-----+--------+-----------+
10 rows in set (0.00 sec)

limit

  • 限制查询记录的个数

  • limit offset, size
    • offset 表示起始行数, 第一行对应的offset是0
    • size 表示查询记录的个数

mysql> select * from employee;
+----+---------+--------+-----+--------+-----------+
| id | name    | gender | age | salary | depart_id |
+----+---------+--------+-----+--------+-----------+
|  1 | alpha   | male   |  35 |  12000 |         1 |
|  2 | bravo   | male   |  28 |  10000 |         1 |
|  3 | charlie | female |  22 |   7000 |         1 |
|  4 | delta   | female |  37 |  14000 |         1 |
|  5 | echo    | male   |  24 |   8000 |         2 |
|  6 | foxtrot | female |  42 |  20000 |         2 |
|  7 | golf    | female |  29 |  11000 |         2 |
|  8 | hotel   | male   |  31 |  13000 |         2 |
|  9 | india   | male   |  40 |  18000 |         3 |
| 10 | juliet  | male   |  21 |   6000 |         3 |
+----+---------+--------+-----+--------+-----------+
10 rows in set (0.00 sec)

mysql> select * from employee limit 2, 3;
+----+---------+--------+-----+--------+-----------+
| id | name    | gender | age | salary | depart_id |
+----+---------+--------+-----+--------+-----------+
|  3 | charlie | female |  22 |   7000 |         1 |
|  4 | delta   | female |  37 |  14000 |         1 |
|  5 | echo    | male   |  24 |   8000 |         2 |
+----+---------+--------+-----+--------+-----------+
3 rows in set (0.00 sec)

使用顺序

  • where > group by > having > order by > limit

多表操作

外键

  • 外键是表中的一个字段, 这个字段与另外一个表中的字段相匹配
  • 作用
    • 减少占用的空间
    • 方便后期修改

一对多

  • constraint 外键名 foreign key (被约束的字段) references 表名(字段名)
mysql> select * from employee;
+----+---------+--------+-----+--------+-----------+
| id | name    | gender | age | salary | depart_id |
+----+---------+--------+-----+--------+-----------+
|  1 | alpha   | male   |  35 |  12000 |         1 |
|  2 | bravo   | male   |  28 |  10000 |         1 |
|  3 | charlie | female |  22 |   7000 |         1 |
|  4 | delta   | female |  37 |  14000 |         1 |
|  5 | echo    | male   |  24 |   8000 |         2 |
|  6 | foxtrot | female |  42 |  20000 |         2 |
|  7 | golf    | female |  29 |  11000 |         2 |
|  8 | hotel   | male   |  31 |  13000 |         2 |
|  9 | india   | male   |  40 |  18000 |         3 |
| 10 | juliet  | male   |  21 |   6000 |         3 |
+----+---------+--------+-----+--------+-----------+
10 rows in set (0.00 sec)

mysql> select * from department;
+----+------+
| id | dept |
+----+------+
|  1 | RD   |
|  2 | IT   |
|  3 | HR   |
+----+------+
3 rows in set (0.00 sec)

# 添加外键
mysql> alter table employee add constraint fk_employee_dept foreign key (depart_id) references department(id);
Query OK, 10 rows affected (0.04 sec)
Records: 10  Duplicates: 0  Warnings: 0


# 查看两表描述, depaet_id 和 id 的 key列变成了MUL 
mysql> desc employee;
+-----------+-----------------------+------+-----+---------+----------------+
| Field     | Type                  | Null | Key | Default | Extra          |
+-----------+-----------------------+------+-----+---------+----------------+
| id        | int(10) unsigned      | NO   | PRI | NULL    | auto_increment |
| name      | char(10)              | NO   |     |         |                |
| gender    | enum('female','male') | NO   |     | male    |                |
| age       | int(11)               | NO   |     | 0       |                |
| salary    | float                 | NO   |     | 0       |                |
| depart_id | int(10) unsigned      | NO   | MUL | 1       |                |
+-----------+-----------------------+------+-----+---------+----------------+
6 rows in set (0.00 sec)

mysql> desc department;
+-------+------------------+------+-----+---------+----------------+
| Field | Type             | Null | Key | Default | Extra          |
+-------+------------------+------+-----+---------+----------------+
| id    | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| dept  | char(10)         | NO   |     |         |                |
+-------+------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

多对多


mysql> select * from boy;
+----+----------+
| id | boy_name |
+----+----------+
|  1 | Jack Ma  |
|  2 | Pony Ma  |
|  3 | Robin Li |
+----+----------+
3 rows in set (0.00 sec)

mysql> select * from girl;
+----+------------+
| id | girl_name  |
+----+------------+
|  1 | Mingzhu Do |
|  2 | Jean Liu   |
|  3 | Cathy Meng |
+----+------------+
3 rows in set (0.00 sec)


mysql> create table boy2girl (
    ->id int unsigned auto_increment primary key,
    ->boy_id int(10) unsigned not null default 1,
    ->girl_id int(10) unsigned not null default 1,
    ->constraint fk_boy2girl_boy foreign key (boy_id) references boy(id),
    ->constraint fk_boy2girl_girl foreign key (girl_id) references girl(id)
    ->);
Query OK, 0 rows affected (0.02 sec)

mysql> desc  boy2girl;
+---------+------------------+------+-----+---------+----------------+
| Field   | Type             | Null | Key | Default | Extra          |
+---------+------------------+------+-----+---------+----------------+
| id      | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| boy_id  | int(10) unsigned | NO   | MUL | 1       |                |
| girl_id | int(10) unsigned | NO   | MUL | 1       |                |
+---------+------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> insert into boy2girl (boy_id, girl_id) values (1,1),(1,2),(1, 3),(2, 2), (2,3),(3,3);
Query OK, 6 rows affected (0.00 sec)
Records: 6  Duplicates: 0  Warnings: 0

# 分别对两个字段进行了限制
mysql> select * from boy2girl;
+----+--------+---------+
| id | boy_id | girl_id |
+----+--------+---------+
|  1 |      1 |       1 |
|  2 |      1 |       2 |
|  3 |      1 |       3 |
|  4 |      2 |       2 |
|  5 |      2 |       3 |
|  6 |      3 |       3 |
+----+--------+---------+
6 rows in set (0.00 sec)

一对一

mysql> select * from employee1;
+----+---------+
| id | name    |
+----+---------+
|  1 | alpha   |
|  2 | bravo   |
|  3 | charlie |
+----+---------+
3 rows in set (0.00 sec)

mysql> create table salary (
    -> id int unsigned auto_increment primary key,
    -> salary float not null default 0,
    -> employee_id int(10) unsigned not null default 1,
    -> constraint fk_employee_salary foreign key (employee_id) references employee1(id)
    -> );
Query OK, 0 rows affected (0.02 sec)


mysql> select * from salary;
+----+--------+-------------+
| id | salary | employee_id |
+----+--------+-------------+
|  1 |  10000 |           1 |
|  2 |  12000 |           2 |
|  3 |  14000 |           3 |
+----+--------+-------------+
3 rows in set (0.00 sec)

多表联查

  • left join on 左外连接, 优先显示左边表的全部记录
  • right join on 右外连接, 优先显示左边表的全部记录
  • inner join on 全外连接, 显示左右两表的全部记录

mysql> select * from boy2girl left join boy on boy_id = boy.id left join girl on girl_id = girl.id;
+----+--------+---------+------+----------+------+------------+
| id | boy_id | girl_id | id   | boy_name | id   | girl_name  |
+----+--------+---------+------+----------+------+------------+
|  1 |      1 |       1 |    1 | Jack Ma  |    1 | Mingzhu Do |
|  2 |      1 |       2 |    1 | Jack Ma  |    2 | Jean Liu   |
|  3 |      1 |       3 |    1 | Jack Ma  |    3 | Cathy Meng |
|  4 |      2 |       2 |    2 | Pony Ma  |    2 | Jean Liu   |
|  5 |      2 |       3 |    2 | Pony Ma  |    3 | Cathy Meng |
|  6 |      3 |       3 |    3 | Robin Li |    3 | Cathy Meng |
+----+--------+---------+------+----------+------+------------+
6 rows in set (0.00 sec)

mysql> select boy_name,girl_name from boy2girl left join boy on boy_id = boy.id left join girl on girl_id = girl.id;
+----------+------------+
| boy_name | girl_name  |
+----------+------------+
| Jack Ma  | Mingzhu Do |
| Jack Ma  | Jean Liu   |
| Jack Ma  | Cathy Meng |
| Pony Ma  | Jean Liu   |
| Pony Ma  | Cathy Meng |
| Robin Li | Cathy Meng |
+----------+------------+
6 rows in set (0.00 sec)

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