left join与inner join

举例a、b表：

bId	bNum
1	20
2	30

aId	aNum
1	10
2	20
3	30

left join以左表为准

select * from a left join b on a.aId=b.bId;

aId	aNum	bId	bNum
1	10	1	20
2	20	2	30
3	30	NULL	NULL

inner join 选两张表都有的，一一相等的

aId	aNum	bId	bNum
1	10	1	20
2	20	2	30