MySQL学习笔记07_多表查询(上)

等值连接

备注

1. 结果集为多表的交集部分
2. n表连接,至少需要n-1个连接条件
3. 表的顺序没有要求

示例

查询有奖金的员工名,部门名

SELECT name, dept_name, bonus
FROM emp, dept
WHERE emp.id = dept.id
AND emp.bonus IS NOT NULL;

查询每个城市的部门个数

SELECT COUNT(*), city
FROM dept AS d, location AS l
WHERE d.loc_id = l.id
GROUP BY city

查询每个工种的工种名和员工数,按员工数降序排序

SELECT COUNT(*), job_name
FROM emp, job
WHERE emp.job_id = job.id
ORDER BY COUNT(*) DESC;

查询员工名,部门名和部门所在城市

SELECT emp.name, dept.name, city
FROM emp, dept, location
WHERE emp.dept_id = dept.id
AND dept.loc_id = location.id

非等值连接

示例

查询员工的工资和工资级别

SELECT salary, grade_level
FROM emp, salary_grade g
WHERE salary BETWEEN g.lowest_sal AND g.highest_sal;

自连接

示例

查询员工名及其上级名

SELECT e.name, m.name manager
FROM emp e, emp m
WHERE e.manager_id = m.id;