连接查询：
- 也叫多表查询。常用于查询字段来自于多张表
- 如果直接查询两张表，将会得到笛卡尔积
mysql> select name, dept_name from employees, departments;
- 通过添加有效的条件可以进行查询结果的限定
mysql> select name, dept_name from employees, departments where employees.dept_id=departments.dept_id;
连接分类
按功能分类
- 内连接(重要)
- 等值连接
- 非等值连接
- 自连接
- 外连接
- 左外连接(重要)
- 右外连接(重要)
- 全外连接(mysql不支持，可以使用UNION实现相同的效果)
- 交叉连接
按年代分类
- SQL92标准：仅支持内连接
- SQL99标准：支持所功能的连接
SQL99标准多表查询
- 语法格式
SELECT 字段...
FROM 表1 [AS] 别名 [连接类型]
JOIN 表2 [AS] 别名
ON 连接条件
WHERE 分组前筛选条件
GROUP BY 分组
HAVING 分组后筛选条件
ORDER BY 排序字段
内连接
- 语法格式
select 查询列表
from 表1 别名
inner join 表2 别名 on 连接条件
inner join 表3 别名 on 连接条件
[where 筛选条件]
[group by 分组]
[having 分组后筛选]
[order by 排序列表]
等值连接
- 查询每个员工所在的部门名
mysql> select name, dept_name
-> from employees
-> inner join departments
-> on employees.dept_id=departments.dept_id;
- 查询每个员工所在的部门名，使用别名
mysql> select name, dept_name
-> from employees as e
-> inner join departments as d
-> on e.dept_id=d.dept_id;
- 查询每个员工所在的部门名，使用别名。两个表中的同名字段，必须指定表名
mysql> select name, d.dept_id, dept_name
-> from employees as e
-> inner join departments as d
-> on e.dept_id=d.dept_id;
- 查询11号员工的名字及2018年每个月工资
mysql> select name, date, basic+bonus as total
-> from employees as e
-> inner join salary as s
-> on e.employee_id=s.employee_id
-> where year(s.date)=2018 and e.employee_id=11;
- 查询2018年每个员工的总工资
mysql> select name, sum(basic+bonus) from employees
-> inner join salary
-> on employees.employee_id=salary.employee_id
-> where year(s.date)=2018
-> group by name;
- 查询2018年每个员工的总工资，按工资升序排列
mysql> select name, sum(basic+bonus) as total from employees as e
-> inner join salary as s
-> on e.employee_id=s.employee_id
-> where year(s.date)=2018
-> group by name
-> order by total;
- 查询2018年总工资大于30万的员工，按工资降序排列
mysql> select name, sum(basic+bonus) as total from employees as e
-> inner join salary as s
-> on e.employee_id=s.employee_id
-> where year(s.date)=2018
-> group by name
-> having total>300000
-> order by total desc;
非等值连接
附：创建工资级别表
创建表语法：
CREATE TABLE 表名称
(
列名称1 数据类型,
列名称2 数据类型,
列名称3 数据类型,
....
)
创建工资级别表：
id：主键。仅作为表的行号
grade：工资级别，共ABCDE五类
low：该级别最低工资
high：该级别最高工资
mysql> use nsd2021;
mysql> create table wage_grade
-> (
-> id int,
-> grade char(1),
-> low int,
-> high int,
-> primary key (id));
向表中插入数据：
- 语法：
INSERT INTO 表名称 VALUES (值1, 值2,....);
向wage_grade表中插入五行数据：
mysql> insert into wage_grade values
-> (1, 'A', 5000, 8000),
-> (2, 'B', 8001, 10000),
-> (3, 'C', 10001, 15000),
-> (4, 'D', 15001, 20000),
-> (5, 'E', 20001, 1000000);
- 查询2018年12月员工基本工资级别
mysql> select employee_id, date, basic, grade
-> from salary as s
-> inner join wage_grade as g
-> on s.basic between g.low and g.high
-> where year(date)=2018 and month(date)=12;
- 查询2018年12月员工各基本工资级别的人数
mysql> select grade, count(*)
-> from salary as s
-> inner join wage_grade as g
-> on s.basic between g.low and g.high
-> where year(date)=2018 and month(date)=12
-> group by grade;
+-------+----------+
| grade | count(*) |
+-------+----------+
| A | 13 |
| B | 12 |
| C | 30 |
| D | 32 |
| E | 33 |
+-------+----------+
5 rows in set (0.00 sec)
- 查询2018年12月员工基本工资级别，员工需要显示姓名
mysql> select name, date, basic, grade
-> from employees as e
-> inner join salary as s
-> on e.employee_id=s.employee_id
-> inner join wage_grade as g
-> on s.basic between g.low and g.high
-> where year(date)=2018 and month(date)=12;
自连接
-
要点：
- 将一张表作为两张使用
- 每张表起一个别名
查看哪些员的生日月份与入职月份相同
mysql> select e.name, e.hire_date, em.birth_date
-> from employees as e
-> inner join employees as em
-> on month(e.hire_date)=month(em.birth_date)
-> and e.employee_id=em.employee_id;
+-----------+------------+------------+
| name | hire_date | birth_date |
+-----------+------------+------------+
| 李玉英 | 2012-01-19 | 1974-01-25 |
| 郑静 | 2018-02-03 | 1997-02-14 |
| 林刚 | 2007-09-19 | 1990-09-23 |
| 刘桂兰 | 2003-10-14 | 1982-10-11 |
| 张亮 | 2015-08-10 | 1996-08-25 |
| 许欣 | 2011-09-09 | 1982-09-25 |
| 王荣 | 2019-11-14 | 1999-11-22 |
+-----------+------------+------------+
7 rows in set (0.00 sec)
外连接
常用于查询一个表中有，另一个表中没有的记录
如果从表中有和它匹配的，则显示匹配的值
如j要从表中没有和它匹配的，则显示NULL
外连接查询结果=内连接查询结果+主表中有而从表中没有的记录
左外连接中，left join左边的是主表
右外连接中，right join右边的是主表
左外连接和右外连接可互换，实现相同的目标
左外连接
- 语法
SELECT tb1.字段..., tb2.字段
FROM table1 AS tb1
LEFT OUTER JOIN table2 AS tb2
ON tb1.字段=tb2.字段
- 查询所有部门的人员以及没有员工的部门
mysql> select d.*, e.name
-> from departments as d
-> left outer join employees as e
-> on d.dept_id=e.dept_id;
右外连接
- 语法
SELECT tb1.字段..., tb2.字段
FROM table1 AS tb1
RIGHT OUTER JOIN table2 AS tb2
ON tb1.字段=tb2.字段
- 查询所有部门的人员以及没有员工的部门
mysql> select d.*, e.name
-> from employees as e
-> right outer join departments as d
-> on d.dept_id=e.dept_id;
交叉连接
- 返回笛卡尔积
- 语法：
SELECT <字段名> FROM <表1> CROSS JOIN <表2> [WHERE子句]
- 查询员工表和部门表的笛卡尔积
mysql> select name, dept_name
-> from employees
-> cross join departments;