left join和right join和inner join

此圖僅限於理解他們之間的關系,下面還有舉例,例子更好明白.

    left join(左聯接) 返回包括左表中的所有記錄和右表中聯結字段相等的記錄 
　right join(右聯接) 返回包括右表中的所有記錄和左表中聯結字段相等的記錄
　inner join(等值連接) 只返回兩個表中聯結字段相等的行

下圖是t1和t2兩張表格

 

t1表格

 

t2表格

 

#1.
select * from
t1 a left join t2 b
on a.L1 = b.L2;

 

#2.
select * from
t2 a right join t1 b
on a.L2 = b.L1;

 

#3.
select * from
t2 a left join t1 b on a.L2 = b.L1;

 

#4.
select * from
t2 a inner join t1 b
on a.L2 = b.L1;

 

#5.
select * from
t2 a,t1 b
where a.L2 = b.L1;

 

#6.
select * from
t1 a left join t2 b
on a.L1 = b.L2
where a.L1 = b.L2;

 

#7.
select * from
t1 a left join t2 b
on a.L1 = b.L2
where a.L1 = null;

 

#8.
select * from
t1 a left join t2 b
on a.L1 = b.L2
where b.L2 is null;

 

#9.
select * from
t1 a left join t2 b
on a.L1 = b.L2
where b.L2 is not null;

 

#10.
select * from
t1 a left join t2 b
on a.L1 = b.L2
where a.L1 = 1;

 

 

#11.
select * from
t1 a left join t2 b
on a.L1 = b.L2
where b.L2 = 1;