舉例a、b表：
| bId | bNum |
| 1 | 20 |
| 2 | 30 |
| aId | aNum |
| 1 | 10 |
| 2 | 20 |
| 3 | 30 |
left join以左表為准
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 選兩張表都有的，一一相等的
| aId | aNum | bId | bNum |
| 1 | 10 | 1 | 20 |
| 2 | 20 | 2 | 30 |
left join和inner join的區別
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