假如有如下三张表：
table1:
| id | field1 |
| 1 | v11 |
| 2 | v22 |
table2:
| id | field2 |
| 1 | v21 |
| 2 | v22 |
| 3 | v23 |
| 4 | v24 |
table3:
| id | field3 |
| 1 | v31 |
| 2 | v32 |
| 3 | v33 |
| 4 | v34 |
| 5 | v35 |
| 6 | v36 |
三表联合查询：
select A.field, D.field2, D.field3
from table1 A left join (
select B.id, B.field2, C.field3 from table2 B left join table3 C on B.id = C.id
) D on A.id = D.id
返回结果：
| id | field1 | field2 | field3 |
| 1 | v11 | v21 | v31 |
| 2 | v12 | v22 | v32 |
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