#sql99语法
#1.内连接
/*
语法：
select 查询列表
from 表1 别名
inner join 表2 别名
on 连接条件;
分类：
等值连接
非等值连接
自连接
*/
#1.等值连接
#查询员工名、部门名
SELECT last_name,department_name
FROM departments d
INNER JOIN employees e
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(*) 部门个数
FROM departments d
INNER JOIN locations l
ON d.location_id=l.location_id
GROUP BY city
HAVING COUNT(*) > 3;
#查询哪个部门的员工个数>3的部门名和员工个数，并按个数降序（添加排序）
SELECT COUNT(*),department_name
FROM employees e
INNER JOIN departments d
ON e.department_id=d.department_id
GROUP BY department_name
HAVING COUNT(*)
ORDER BY COUNT(*) DESC;
#查询员工名，部门名，工种名，并按部门名降序
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
#查询员工的工资级别
SELECT salary,grade_level
FROM employees e
INNER 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，并且按工资级别降序
SELECT COUNT(*),grade_level
FROM employees e
INNER JOIN job_grades j
ON e.salary BETWEEN j.lowest_sal AND j.highest_sal
GROUP BY grade_level
HAVING COUNT(*) > 20
ORDER BY grade_level DESC;
#自连接
#查询员工的名字、上级的名字
SELECT e.last_name,m.last_name
FROM employees e
INNER JOIN employees m
WHERE e.employee_id=m.manager_id;
#二、外连接
SELECT * FROM beauty;
SELECT * FROM boys;
/*
特点：
1、外连接的查询结果为主表的所有记录
如果从表中有和它匹配的，则显示匹配的值
如果从表中没有和它匹配的，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结果
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
*/
#引入：查询没有男朋友的女神名
SELECT b.name,bo.*
FROM beauty b
LEFT OUTER JOIN boys bo
ON b.boyfriend_id=bo.id
#右外连接
SELECT b.name,bo.*
FROM boys bo
RIGHT OUTER JOIN beauty b
ON b.boyfriend_id=bo.id
WHERE bo.id IS NULL;
#查询哪个部门没有员工
#左外
SELECT d.*,e.employee_id
FROM departments d
LEFT OUTER JOIN employees e
ON d.department_id=e.department_id
WHERE e.employee_id IS NULL;