多表查询
含义：当查询的字段来自多个表的时候，就会用到多表查询
笛卡尔迪 现象：表1 有n行，表2有m行，结果m*n行；
发生原因：没有有效的连接条件
分类：
内连接：
等值连接
非等值连接
自连接
外链接：
左外连接
右外连接
全外连接
交叉连接：
92 语法：
SELECT * FROM beauty;
SELECT * FROM boys;
SELECT NAME,boyName FROM boys,beauty
WHERE beauty.`boyfriend_id`=boys.`id`;
# 1. 等值连接
特点：1.多表等值连接的结果为多表的交集部分
2.n表连接，至少需要n-1个连接条件
3.多表的顺序没有要求
4.一般需要为表起别名；
SELECT NAME,boyName
FROM boys,beauty
WHERE beauty.boyfriend_id=boys.id;
# 查询有奖金的员工名和部门名
SELECT last_name ,department_name
FROM employees e,departments d
WHERE e.`department_id`=d.`department_id`
AND e.`commission_pct` IS NOT NULL;
# 查询城市名中第二个字符为o的部门名和城市名
SELECT department_name, city
FROM departments d,locations l
WHERE d.`location_id`=l.`location_id`
AND city LIKE "_o%";
#查询每个城市的部门个数
SELECT COUNT(*) ,city
FROM departments d,locations l
WHERE d.`location_id`=l.`location_id`
GROUP BY city;
# 查询每个工种的名成和员工的个数，并按员工个数降序
SELECT job_title,COUNT(*)
FROM employees e,jobs j
WHERE e.`job_id`=j.`job_id`
GROUP BY job_title
ORDER BY COUNT(*) DESC;
# 查询 员工名、部门名和所在的城市
SELECT last_name ,department_name,city
FROM employees e,departments d,locations l
WHERE e.`department_id`=d.`department_id`
AND d.`location_id`=l.`location_id`
多表查询
# 多表查询的非等值连接
SELECT salary ,grade_level
FROM employees e,job_grades j
WHERE salary BETWEEN j.`lowest_sal` AND j.`highest_sal`
AND j.grade_level ='A';
# 自连接
#查询员工名和上级名称
SELECT e.`last_name`,m.`last_name`
FROM employees e,employees m
WHERE e.`manager_id`=m.`employee_id`;
99语法
# 1. 内连接
语法：
select 列名
from 表1 别名
inner join 表2 别名
on 连接条件；
分类：等值，非等值，自连接
特点：
1.可以添加排序，分组，筛选
2.inner'可以省略
3. 筛选条件方法where后面，连接条件放在on后面，提高阅读
# 等值连接
# 查询员工名和部门名
SELECT last_name ,department_name
FROM employees e
INNER JOIN departments d
ON e.`department_id`=d.`department_id`;
# 查询名字中包含e的员工名和工种名
SELECT last_name ,job_title
FROM employees e
INNER JOIN jobs j
ON e.`job_id`=j.`job_id`
WHERE e.`last_name` LIKE '%e%';
# 查询部门个数>3 的城市名和部门个数
SELECT city,COUNT(*)
FROM departments d
INNER JOIN locations l
ON d.`location_id`=l.`location_id`
GROUP BY city
HAVING COUNT(*)>3;
# 查询那个部门的员工个数》3的部门名和员工个数，并按照个数降序
SELECT department_name,COUNT(*)
FROM departments d
INNER JOIN employees e
ON e.`department_id`=d.`department_id`
GROUP BY department_name
HAVING COUNT(*)>3
ORDER BY COUNT(*) DESC;
# 查询员工名，部门名，工种名，并按照部门名降序
SELECT last_name ,department_name ,job_title
FROM employees e
INNER JOIN departments d ON e.`department_id`=d.`department_id`
INNER JOIN jobs j ON e.`job_id`=j.`job_id`
ORDER BY department_name DESC;
# 非等值连接
# 查询员工工资的级别
SELECT salary ,grade_level
FROM employees e
JOIN job_grades g
ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`;
# 查询工资级别的个数>20 的个数，并且按照工资级别降序
SELECT COUNT(*) ,grade_level
FROM employees e
JOIN job_grades g
ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`
GROUP BY grade_level
HAVING COUNT(*)>20
ORDER BY grade_level DESC;
# 自连接
#查询员工的名字和上级的名字
SELECT e.last_name,m.last_name
FROM employees e
JOIN employees m
ON e.`manager_id`=m.`employee_id`;
#查询员工名包含k的员工名和上级的名字
SELECT e.last_name,m.last_name
FROM employees e
JOIN employees m
ON e.`manager_id`=m.`employee_id`
WHERE e.`last_name` LIKE "%k%";
外链接
应用场景：用于查询一个表中有，另一个表中没有的记录
特点：
1.外链接查询的结果为主表中的所有记录
如果从表中和它匹配的，则显示匹配的值
如果从表中没有匹配到，则显示null
外连接查询结果=内连接结果+主表中有而从表中没有的记录
2.左外连接，left join 左边的是主表
右外连接，right join 右边的是主表
3.左外和右外交换两个表的顺序，可以实现同样的效果
4.全外连接，内连接的结果+表1中有但是表2中没有的+表2中有但是表1中没有的；
# 引入：查询男朋友不在男神表中的女神名
# 左外连接
SELECT b.`name`,bo.`boyName`
FROM beauty b
LEFT OUTER JOIN boys bo
ON b.`boyfriend_id`=bo.`id`
WHERE bo.`id` IS NULL;
# 左外连接
SELECT b.`name`,bo.`boyName`
FROM boys bo
RIGHT OUTER JOIN beauty b
ON b.`boyfriend_id`=bo.`id`
WHERE bo.`id` IS NULL;
# 查询那个部门没有员工
# 左外
SELECT d.* ,e.*
FROM departments d
LEFT OUTER JOIN employees e
ON d.`department_id`=e.`department_id`
WHERE e.`employee_id` IS NULL;
# 全外连接
SELECT b.*,bo.*
FROM beauty b
FULL OUTER JOIN boys bo
ON b.`boyfriend_id`= bo.id;
交叉连接
# 交叉连接（笛卡尔乘积）
SELECT b.* ,bo.*
FROM beauty b
CROSS JOIN boys bo;