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SELECT N.a,M.f1,N.b FROM (SELECT A.b,B.b AS f1 FROM A LEFT JOIN B ON B.a=A.a) AS M LEFT JOIN B AS N ON M.b=N.a
mysql> select * from a;
+------+------+
| a | b |
+------+------+
| 1 | 2 |
| 3 | 3 |
| 3 | 1 |
| 2 | 3 |
+------+------+
4 rows in set (0.00 sec)
mysql> select * from b;
+------+------+
| a | b |
+------+------+
| 1 | A1 |
| 2 | A2 |
| 3 | A3 |
| 4 | A4 |
+------+------+
4 rows in set (0.00 sec)
mysql> select B.b,C.b From a A left join b B on A.a=B.a Left join b C on A.b=C.a
;
+------+------+
| b | b |
+------+------+
| A1 | A2 |
| A3 | A3 |
| A3 | A1 |
| A2 | A3 |
+------+------+
4 rows in set (0.00 sec)
Select B.b,C.b
From a A Left Join b B on A.a=B.a
Left Join b C on A.b=C.a