The difference between left join and inner join

 

Example a, b table:

bId	bNum
1	20
2	30

 

aId	aNum
1	10
2	20
3	30

 

left join is subject to the left table

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 selects both tables, which are equal one by one

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