4.1分表查询（等值连接）
FROM employees,departments
WHERE employees.department_id=departments.department_id;
#案例3：查询员工名，工种号，工种名
#为表名起别名，简洁，一旦使用了别名，则不能使用原来的表名了。
SELECT last_name,e.job_id,job_title
FROM employees e,jobs j
WHERE e.job_id = j.job_id;
#4，可以加筛选吗？
#案例；查询有奖金的员工名，部门名
SELECT last_name,department_name
FROM employees e,departments d
WHERE e.department_id = e.department_id && e.commission_pct is not null;
#5,查询城市名中第二个字符为o的部门名和城市名
# 可以添加筛选条件的。
SELECT department_name,city
FROM departments d,locations l
WHERE d.location_id = l.location_id && l.city like '_o%';
#可以添加分组吗？
#案例1：查询每个城市的部门个数
SELECT COUNT(*) 个数,city
FROM departments d,locations l
WHERE d.location_id = l.location_id
GROUP BY city;
#案例2
#查询有奖金的每一个部门名和部门领导编号，和该部门的最低工资
SELECT d.department_name,MIN(e.salary)
FROM departments d,employees e
WHERE d.department_id = e.department_id && e.commission_pct is not NULL
GROUP BY department_name;
#案例3：查询每个工种名和员工个数，并且按照员工个数降序。
SELECT job_title,COUNT(*)
FROM employees e,jobs j
WHERE e.job_id = j.job_id
GROUP BY job_title
ORDER BY COUNT(*) DESC;
4.2分表查询（非等值连接）
#2,非等值查询
#案例1，查询员工的工资和工资级别
SELECT *
FROM job_grades;
SELECT salary,grade_level
FROM employees e,job_grades g
WHERE salary BETWEEN g.lowest_sal AND g.highest_sal;
4.3表查询（相当于多次查找本表）（自连接）
