Mysql连表查询

1.等值连接
案例1：查询员工名、部门名
select last_name,department_name
from employees e 
inner join departments d
on e.department_id=d.department_id;
案例2：查询名字中包含e的员工名和工种名(添加筛选)
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%";
案例3：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;
案例4：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
select count(*) 个数,department_name
from employees e
inner join departments d
on e.department_id=d.department_id
group by department_name
having count(*)>3
order by count(*) desc;
案例5：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;
2.非等值连接
案例：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
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;
3.自连接
select e.last_name,m.last_name
from employees e
join employees m
on e.manager_id=m.manager_id
where e.last_name like "%k%";

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;
#案例：查询女神中有男朋友的男朋友信息
select b.name,bo.* from boys bo left outer join beauty b on b.boyfriend_id=bo.id;
#案例：查询编号>3的女神的男朋友信息，如果有则列出详细，如果没有，用null填充
select b.id,b.name,bo.* from beauty b left join boys bo on b.boyfriend_id=bo.id where b.id>3;

select b.*,bo.*
from beauty b
cross join boys bo;