mysql 多表查询

'''
1.笛卡尔集
'''
select *
from worker,location

'''
2.内连接
'''
select *
from worker inner join location
on worker.country=location.country;

'''
3.左外连接，右外连接
'''
select *
from worker left join location
on worker.country=location.country;

'''
4.union实现外连接
'''
select *
from worker left join location
on worker.country=location.country
union
select *
from worker right join location
on worker.country=location.country;