MySQL Full Join等价改写

MySQL不支持Full Join，可以这样改写：

```bash
select * from t1 full join t2 on t1.id = t2.id;
```

等价改写 =>

```bash
select * from t1 left join t2 on t1.id = t2.id
union all
select * from t1 right join t2 on t1.id = t2.id where t1.id is null;
```