多表查询

多表查询

新建表

mysql> create table employee6(                                                                                
    -> emp_id int auto_increment primary key,
    -> emp_name varchar(50),
    -> age int,
    -> dept_id int);
Query OK, 0 rows affected (0.18 sec)

mysql> create table department6(
    -> dept_id int,
    -> dept_name varchar(50));
Query OK, 0 rows affected (0.17 sec)

插入数据

mysql> insert into employee6(emp_name,age,dept_id) values
    -> ('tianyun',19,200),
    -> ('tom',26,201),
    -> ('jack',30,202),
    -> ('alice',24,203),
    -> ('robin',40,204),
    -> ('narasha',28,205);
Query OK, 6 rows affected (0.01 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> insert into department6 values(200,'hr'), (201,'it'), (202,'sale'), (203,'fd');
Query OK, 4 rows affected (0.01 sec)
Records: 4  Duplicates: 0  Warnings: 0

查看表

mysql> select * from employee6;
+--------+----------+------+---------+
| emp_id | emp_name | age  | dept_id |
+--------+----------+------+---------+
|      1 | tianyun  |   19 |     200 |
|      2 | tom      |   26 |     201 |
|      3 | jack     |   30 |     202 |
|      4 | alice    |   24 |     203 |
|      5 | robin    |   40 |     204 |
|      6 | narasha  |   28 |     205 |
+--------+----------+------+---------+
6 rows in set (0.00 sec)

mysql> select * from department6;
+---------+-----------+
| dept_id | dept_name |
+---------+-----------+
|     200 | hr        |
|     201 | it        |
|     202 | sale      |
|     203 | fd        |
+---------+-----------+
4 rows in set (0.00 sec)

内连接(只连接匹配的行)

mysql>select employee6.emp_id,employee6.emp_name, employee6.age, department6.dept_name
    ->from employee6, department6 
    ->where employee66.dept_id = department6.dept_id;
+--------+----------+------+-----------+
| emp_id | emp_name | age  | dept_name |
+--------+----------+------+-----------+
|      1 | tianyun  |   19 | hr        |
|      2 | tom      |   26 | it        |
|      3 | jack     |   30 | sale      |
|      4 | alice    |   24 | fd        |
+--------+----------+------+-----------+
4 rows in set (0.00 sec)

可以简写为,但是如果该字段两张表都存在,则不能简写

mysql> select emp_id,emp_name,age,dept_name from employee6, department6 where employee6.dept_id = department6.dept_id;
+--------+----------+------+-----------+
| emp_id | emp_name | age  | dept_name |
+--------+----------+------+-----------+
|      1 | tianyun  |   19 | hr        |
|      2 | tom      |   26 | it        |
|      3 | jack     |   30 | sale      |
|      4 | alice    |   24 | fd        |
+--------+----------+------+-----------+
4 rows in set (0.00 sec)

发现只显示了有部门的员工

外连接

左连接 以left join左面的表为准

mysql> select emp_id,emp_name,age,dept_name from employee6 left join department6 on emplooyee6
+--------+----------+------+-----------+
| emp_id | emp_name | age  | dept_name |
+--------+----------+------+-----------+
|      1 | tianyun  |   19 | hr        |
|      2 | tom      |   26 | it        |
|      3 | jack     |   30 | sale      |
|      4 | alice    |   24 | fd        |
|      5 | robin    |   40 | NULL      |
|      6 | narasha  |   28 | NULL      |
+--------+----------+------+-----------+
6 rows in set (0.00 sec)

右连接,以right join左面的表为准

mysql> insert into department6 values(510,'os');
Query OK, 1 row affected (0.04 sec)

mysql> select emp_id,emp_name,age,dept_name from employee6 right join department6 on emplloyee6
+--------+----------+------+-----------+
| emp_id | emp_name | age  | dept_name |
+--------+----------+------+-----------+
|      1 | tianyun  |   19 | hr        |
|      2 | tom      |   26 | it        |
|      3 | jack     |   30 | sale      |
|      4 | alice    |   24 | fd        |
|   NULL | NULL     | NULL | os        |
+--------+----------+------+-----------+
5 rows in set (0.00 sec)

复合查询

以内连接的方式查询employee6和department6表,并且employee6表中的age字段值需要大于25

找出公司部门中年龄大于25岁的员工

mysql> select emp_id,emp_name,age,dept_name from employee6, department6 where employee6.dept_id = department6.dept_id and age>=25;
+--------+----------+------+-----------+
| emp_id | emp_name | age  | dept_name |
+--------+----------+------+-----------+
|      2 | tom      |   26 | it        |
|      3 | jack     |   30 | sale      |
+--------+----------+------+-----------+
2 rows in set (0.00 sec)

以内连接的方式查询employee6和department6表,并且以age字段的升序显示

mysql> select emp_id,emp_name,age,dept_name from employee6, department6 where employee6.dept_id = department6.dept_id order by age;
+--------+----------+------+-----------+
| emp_id | emp_name | age  | dept_name |
+--------+----------+------+-----------+
|      1 | tianyun  |   19 | hr        |
|      4 | alice    |   24 | fd        |
|      2 | tom      |   26 | it        |
|      3 | jack     |   30 | sale      |
+--------+----------+------+-----------+
4 rows in set (0.00 sec)

来源：https://www.cnblogs.com/inmeditation/p/11660757.html