hive：幾個join的差別彙總

employee表

department表

1.笛卡爾積

select * from employee a ,department b;

共4*2=8條結果。

2.inner join

select * from employee a 
inner join department b
on a.DepartmentId=b.id; 

等價於

select * from employee a ,department b
where a.DepartmentId=b.id; 

3.left join

返回滿足連接條件的左邊表的所有記錄，若左邊表的某些記錄在右邊表中沒有匹配記錄，右邊表則顯示null。

（1）如果不指定連接條件，left join的結果將和笛卡爾積的結果相同。

select * from employee a 
left join department b;

（2）employee left join department

select * from employee a 
left join department b
on a.DepartmentId=b.id;

（3）department left join employee

交換一下left join左右兩表的位置：
department left join employee的結果 = employee right join department的結果（顯示位置不同）

select * from department a
left join  employee b 
on a.id=b.departmentId;

4.right join

返回滿足連接條件的右邊表的所有記錄，若右邊表的記錄在左邊表中沒有匹配記錄，左邊表則顯示null。

select * from employee a 
right join department b;

select * from employee a 
right join department b
on a.departmentId=b.id;

等價於

select * from employee a 
left join department b
on a.departmentId=b.id
where b.name is not null;