内连接：INNER JOIN

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数据准备

1.员工表（tb_emp）

CREATE TABLE `tb_emp` (
  `employee_id` int(6) NOT NULL AUTO_INCREMENT COMMENT '员工编号',
  `first_name` varchar(20) DEFAULT NULL COMMENT '名',
  `last_name` varchar(25) DEFAULT NULL COMMENT '性',
  `email` varchar(25) DEFAULT NULL COMMENT '邮箱',
  `phone_number` varchar(20) DEFAULT NULL COMMENT '电话号码',
  `job_id` varchar(10) DEFAULT NULL COMMENT '工种id',
  `salary` double(10,2) DEFAULT NULL COMMENT '工资',
  `commission_pct` double(4,2) DEFAULT NULL COMMENT '奖金率',
  `manager_id` int(6) DEFAULT NULL COMMENT '领导编号',
  `department_id` int(4) DEFAULT NULL COMMENT '部门编号',
  `hiredate` datetime DEFAULT NULL COMMENT '入职日期',
  PRIMARY KEY (`employee_id`)
) ENGINE=InnoDB AUTO_INCREMENT=207 DEFAULT CHARSET=gb2312;

2.部门表（tb_emp）

CREATE TABLE `tb_dep` (
  `department_id` int(4) NOT NULL AUTO_INCREMENT COMMENT '部门编号',
  `department_name` varchar(3) DEFAULT NULL COMMENT '部门名称',
  `manager_id` int(6) DEFAULT NULL COMMENT '部门领导编号',
  `location_id` int(4) DEFAULT NULL COMMENT '位置编号',
  PRIMARY KEY (`department_id`)
) ENGINE=InnoDB AUTO_INCREMENT=271 DEFAULT CHARSET=gb2312;

3.工种表（tb_job）

CREATE TABLE `tb_job` (
  `job_id` varchar(10) NOT NULL COMMENT '工种编号',
  `job_title` varchar(35) DEFAULT NULL COMMENT '工种名称',
  `min_salary` int(6) DEFAULT NULL COMMENT '最低工资',
  `max_salary` int(6) DEFAULT NULL COMMENT '最高工资',
  PRIMARY KEY (`job_id`)
) ENGINE=InnoDB DEFAULT CHARSET=gb2312;

4.工资等级表

CREATE TABLE `tb_job_grade` (
  `grade_level` varchar(3) DEFAULT NULL COMMENT '工资等级',
  `lowest_sal` int(11) DEFAULT NULL COMMENT '等级的最小工资',
  `highest_sal` int(11) DEFAULT NULL COMMENT '等级的最大工资'
) ENGINE=InnoDB DEFAULT CHARSET=gb2312;

1.等值内连接

1.1两张表的等值连接

需求：查询部门的员工个数大于5的部门名称和该部门的员工个数（coun），并按照员工个数按照降序取Top3.

mysql> SELECT department_name,count(1) coun 
       FROM tb_emp e 
       INNER JOIN 
       tb_dep d 
       ON e.department_id = d.department_id 
       GROUP BY department_name 
       HAVING coun>5 
       ORDER BY coun DESC 
       LIMIT 3;
+-----------------+------+
| department_name | coun |
+-----------------+------+
| Shi             |   45 |
| Sal             |   34 |
| Fin             |    6 |
+-----------------+------+
3 rows in set

1.2.多表的等值连接

多张表的连接使用多个inner join 连接即可；
需求：查询员工名、部门名、工种名，并且按照员工名称升序取Top5；
分析：
1.员工名、部门名、工种名分写来自于tb_emp、tb_dep、tb_job，那么需要将这三张表进行连接
2.找出连接条件
3.升序order by asc ,top5即limit 5;

mysql> SELECT last_name,department_name,job_title 
       FROM tb_emp e 
       INNER JOIN tb_dep d 
       ON e.department_id=d.department_id 
       INNER JOIN tb_job j ON e.job_id=j.job_id 
       ORDER BY last_name ASC 
       LIMIT 5;
+-----------+-----------------+---------------------------------+
| last_name | department_name | job_title                       |
+-----------+-----------------+---------------------------------+
| Abel      | Sal             | Sales Representative            |
| Ande      | Sal             | Sales Representative            |
| Atkinson  | Shi             | Stock Clerk                     |
| Austin    | IT              | Programmer                      |
| Baer      | Pub             | Public Relations Representative |
+-----------+-----------------+---------------------------------+
5 rows in se

2.非等值内连接

需求：查询工资等级的个数（coun）大于30的等级名称和个数，并按照个数降序取TOP3
需求分析：
1.等级的个数、等级名称：需要将tb_emp和tb_job_grade进行连接；
2.连接条件：根据工资等级表的各个等级的工资范围判断各个员工的工资在哪一个等级；
3.等级的个数：需要进行分组，并且分组的条件是等级名称；
4.个数降序：order by desc；
5.取出top3：limit 3。

mysql> SELECT COUNT(1) coun ,grade_level 
       FROM tb_emp e 
       JOIN tb_job_grade g 
       ON e.salary BETWEEN g.lowest_sal AND g.highest_sal 
       GROUP BY grade_level 
       HAVING coun > 30 
       ORDER BY coun 
       DESC LIMIT 3;
+------+-------------+
| coun | grade_level |
+------+-------------+
|   38 | C           |
+------+-------------+
1 row in set

3.自连接

需求：查询员工名、以及该员工对应的上级名称
需求分析：
1.员工名和上级名称对来自于tb_emp；
2.连接的条件：员工表1的上级id等于员工表2的员工id

mysql> SELECT e1.last_name ,e2.last_name 
       FROM tb_emp e1 
       JOIN tb_emp e2 ON e1.manager_id = e2.employee_id 
       limit 5; 
+-----------+-----------+
| last_name | last_name |
+-----------+-----------+
| Kochhar   | K_ing     |
| De Haan   | K_ing     |
| Hunold    | De Haan   |
| Ernst     | Hunold    |
| Austin    | Hunold    |
+-----------+--

在sql92中自连接的写法如下：

mysql> SELECT e1.last_name ,e2.last_name
       FROM tb_emp e1,tb_emp e2 
       WHERE e1.manager_id = e2.employee_id 
       limit 5; 
+-----------+-----------+
| last_name | last_name |
+-----------+-----------+
| Kochhar   | K_ing     |
| De Haan   | K_ing     |
| Hunold    | De Haan   |
| Ernst     | Hunold    |
| Austin    | Hunold    |
+-----------+-----------+
5 rows in set

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last edit date：2018-11-18
location:广州-海珠-西畔里