1. 熟悉常见的表
DESC employees;
DESC departments;
DESC locations;
引入：查询一个员工(ABEL)在哪个城市工作
SELECT *
FROM employees
WHERE last_name = 'Abel';
SELECT *
FROM departments
WHERE department_id = 80;
SELECT *
FROM locations
WHERE location_id = 2500;
2. 正确的多表查询
错误的实现方式
SELECT employeea_id,department_name
FROM employees,departments;
正确的实现方式：需要有连接条件
SELECT employee_id,department_name
FROM employees,departments
WHERE employees.`department_id` = departments.`department_id`;
# 着重号可以不写:``
延申:如果查询语句中出现了多个表中都存在的字段,则必须指明此字段所在的表
SELECT employee_id,department_name,departments.department_id
FROM employees,departments
WHERE employees.`department_id` = departments.`department_id`;
建议:都指明字段所在的表
3.可以给表起别名,在select和where中使用
SELECT emp.employee_id,emp.department_name,dept.department_id
FROM employees emp,departments dept
WHERE emp.department_id = dept.department_id;
但是:写了别名以后,一旦在select或者where中使用了别名的话,就必须使用别名!
练习:如果有n个表查询,则至少需要n-1个连接条件
SELECT employee_id,department_name,d.department_id,city
FROM employees e,departments d,locations l
WHERE e.department_id = d.department_id
AND d.location_id = l.location_id;
4.多表查询的分类
- 角度一:
- 等值连接和非等值连接
- 角度二:
- 自连接和非自连接
- 角度三:
- 内连接和外连接
SELECT e.last_name,e.salary,j.grade_level
FROM employees e,job_grades j
WHERE e.`salary` BETWEEN j.`lowest_sal` AND j.`highest_sal`;
练习:查询员工id,员工姓名及其管理者的id和姓名
SELECT emp.employee_id,emp.last_name,mgr.employee_id,mgr.last_name
FROM employees emp,employees mgr
WHERE emp.`manager_id` = mgr.`employee_id`
内连接:合并具有同一列的两个以上的表的行,结果集中不包含一个表与另一个表不匹配的行(即交集)
SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id;
# 只有106条记录
外连接:合并具有同一列的两个以上的表的行,结果集中除了包含一个表与另一个表不匹配的行之外,还查询到了左表或右表中不匹配的行
外连接又分为:(类似于韦恩图)
- 左外连接:59+41
-
左外连接:合并具有同一列的两个以上的表的行,结果集中除了包含一个表与另一个表不匹配的行之外,还查询到了左表中不匹配的行
-
- 右外连接:39+41
- #####右外连接:合并具有同一列的两个以上的表的行,结果集中除了包含一个表与另一个表不匹配的行之外,还查询到了右表中不匹配的行
- 满外连接:59+41+39
-
满连接:合并具有同一列的两个以上的表的行,结果集中除了包含一个表与另一个表不匹配的行之外,还查询到了左表和右表中不匹配的行
-
5.MYSQL中内外连接的实现:
问题:查询所有的员工的last_name,department_name信息.
SQL92语法(可惜的是,MySQL不支持,我们了解就行)
SQL92语法的内连接实现:
SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id;
SQL92语法的外连接实现:使用 +
SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id(+);
SQl99语法(mysql支持这种方式:)
SQl99实现内连接:
SELECT last_name,department_name,city
FROM employees e JOIN departments d
ON e.`department_id` = d.department_id
JOIN locations l
ON d.`location_id` = l.`location_id`;
SQL99实现左外连接(outer可以省略):加上了那些没有部门的人
SELECT last_name,department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;
右外连接:加上了那些没有人的部门
SELECT last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;
满外连接:
但是:MySQL不支持FULL OUTER JOIN
SELECT last_name,department_name
FROM employees e FULL OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;
解决办法:union和union all语句
6.union和union all语句
7种jion的实现:
- 中图:内连接:
SELECT last_name,department_name
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`;
- 左上图:左外
SELECT last_name,department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;
- 右上图:右外
SELECT last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;
- 左中图:
SELECT last_name,department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.department_id IS NULL;
- 右中图
SELECT last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.department_id IS NULL;
6.左下图(满外)
- 方法一.左上图 UNION ALL 右中图
SELECT last_name,department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`
UNION ALL
SELECT last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.department_id IS NULL;
- 方法二.左中图 UNION ALL 右上图
SELECT last_name,department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.department_id IS NULL
UNION ALL
SELECT last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;
7.右下图:左中图 UNION ALL 右中图
SELECT last_name,department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.department_id IS NULL
UNION ALL
SELECT last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.department_id IS NULL;
7. SQL99语法的新特性1：自然连接
可以把自然连接理解为SQL92中的等值连接，他会帮你自动查询两张连接表中的所有相同的字段，然后进行等值连接。
虽然很简洁，但是并不灵活
SELECT employee_id,last_name,department_name
FROM employees e NATURAL JOIN departments d;
8. SQL99语法的新特性2：using的使用
但是这个不适用于自连接！！！
SELECT employee_id,last_name,department_name
FROM employees e JOIN departments d
USING(department_id);
