（七）MySQL数据操作DQL:多表查询2

（1）准备环境

1）创建员工表

mysql> create table company.employee6(
    -> emp_id int auto_increment primary key not null,
    -> emp_name varchar(10),
    -> age int,
    -> dept_id int);
mysql> insert into employee6(emp_name,age,dept_id) values
    -> ('tom',19,200),
    -> ('jack',30,201),
    -> ('alice',24,202),
    -> ('robin',40,200),
    -> ('natasha',28,204);

2）创建部门表

mysql> create table company.department(
    -> dept_id int,
    -> dept_name varchar(100));
mysql> insert into department values (200,'hr'), (201,'it'), (202,'sale'), (203,'fd');

（2）交叉连接

生成笛卡尔积，不使用任何匹配条件
语法：select 表1.字段1,表1.字段2,表2.字段1 from 表1,表2;

mysql> select employee6.emp_name,employee6.age,employee6.dept_id,department.dept_name from employee6,department;
+----------+------+---------+-----------+
| emp_name | age  | dept_id | dept_name |
+----------+------+---------+-----------+
| tom      |   19 |     200 | hr        |
| tom      |   19 |     200 | it        |
| tom      |   19 |     200 | sale      |
| tom      |   19 |     200 | fd        |
| jack     |   30 |     201 | hr        |
| jack     |   30 |     201 | it        |
| jack     |   30 |     201 | sale      |
| jack     |   30 |     201 | fd        |
| alice    |   24 |     202 | hr        |
| alice    |   24 |     202 | it        |
| alice    |   24 |     202 | sale      |
| alice    |   24 |     202 | fd        |
| robin    |   40 |     200 | hr        |
| robin    |   40 |     200 | it        |
| robin    |   40 |     200 | sale      |
| robin    |   40 |     200 | fd        |
| natasha  |   28 |     204 | hr        |
| natasha  |   28 |     204 | it        |
| natasha  |   28 |     204 | sale      |
| natasha  |   28 |     204 | fd        |
+----------+------+---------+-----------+

（3）内连接：根据两张表的相同字段只连接匹配的行

语法：select 表1.字段n,表2.字段n from 表1,表2 表1.字段 = 表2.字段
根据员工表的dept_id 和部门表的dept_id进行连接，只匹配dept_id相同的行

mysql> select employee6.dept_id,employee6.emp_name,employee6.age,department.dept_name from employee6,department where employee6.dept_id = department.dept_id;
+---------+----------+------+-----------+
| dept_id | emp_name | age  | dept_name |
+---------+----------+------+-----------+
|     200 | tom      |   19 | hr        |
|     201 | jack     |   30 | it        |
|     202 | alice    |   24 | sale      |
|     200 | robin    |   40 | hr        |
+---------+----------+------+-----------+

（4）外连接

语法：select 字段列表 from 表1 left|right join 表2 on 表1.字段 = 表2.字段

1）外连接之左连接：会显示左边表内所有的值，不论在右边表内匹不匹配

mysql> select emp_id,emp_name,age,dept_name from employee6 left join department on employee6.dept_id = department.dept_id;
+--------+----------+------+-----------+
| emp_id | emp_name | age  | dept_name |
+--------+----------+------+-----------+
|      1 | tom      |   19 | hr        |
|      4 | robin    |   40 | hr        |
|      2 | jack     |   30 | it        |
|      3 | alice    |   24 | sale      |
|      5 | natasha  |   28 | NULL      |
+--------+----------+------+-----------+

2）外连接之右连接:会显示右边表内所有的值，不论在左边表内匹不匹配

mysql> select emp_id,emp_name,age,dept_name from employee6 right join department on employee6.dept_id = department.dept_id;
+--------+----------+------+-----------+
| emp_id | emp_name | age  | dept_name |
+--------+----------+------+-----------+
|      1 | tom      |   19 | hr        |
|      2 | jack     |   30 | it        |
|      3 | alice    |   24 | sale      |
|      4 | robin    |   40 | hr        |
|   NULL | NULL     | NULL | fd        |
+--------+----------+------+-----------+

（5）复合条件连接查询

以内连接的方式查询 employee6 和 department 表，并且 employee6 表中的 age 字段值必须大于 25，排序

mysql> select emp_id,emp_name,age,dept_name from employee6,department where employee6.dept_id = department.dept_id and age >25;
+--------+----------+------+-----------+
| emp_id | emp_name | age  | dept_name |
+--------+----------+------+-----------+
|      4 | robin    |   40 | hr        |
|      2 | jack     |   30 | it        |
+--------+----------+------+-----------+
2 rows in set (0.00 sec)

mysql> select emp_id,emp_name,age,dept_name from employee6,department where employee6.dept_id = department.dept_id and age >25 order by age;
+--------+----------+------+-----------+
| emp_id | emp_name | age  | dept_name |
+--------+----------+------+-----------+
|      2 | jack     |   30 | it        |
|      4 | robin    |   40 | hr        |
+--------+----------+------+-----------+
2 rows in set (0.00 sec)

mysql> select emp_id,emp_name,age,dept_name from employee6,department where employee6.dept_id = department.dept_id and age >25 order by age desc;
+--------+----------+------+-----------+
| emp_id | emp_name | age  | dept_name |
+--------+----------+------+-----------+
|      4 | robin    |   40 | hr        |
|      2 | jack     |   30 | it        |
+--------+----------+------+-----------+

（6）子查询

子查询是将一个查询语句嵌套在另一个查询语句中。内层查询语句的查询结果，可以为外层查询语句提供查询条件。

1）带in的子查询

mysql> select * from employee6 where dept_id in (select dept_id from department);
+--------+----------+------+---------+
| emp_id | emp_name | age  | dept_id |
+--------+----------+------+---------+
|      1 | tom      |   19 |     200 |
|      2 | jack     |   30 |     201 |
|      3 | alice    |   24 |     202 |
|      4 | robin    |   40 |     200 |
+--------+----------+------+---------+
4 rows in set (0.00 sec)

2）带比较运算符的子查询

mysql> select dept_name from department where dept_id in (select dept_id from employee6 where age >25); 
+-----------+
| dept_name |
+-----------+
| it        |
| hr        |
+-----------+
2 rows in set (0.00 sec)