如何使用连接表来检索数据？

Question

我有表A存储代码，表B存储项目详细信息

TableA                    TableB
--------------------       -------------------------------------
 code_id  code_name         item_id  item_name  code_1   code_2
--------------------       -------------------------------------
    1       AAA                1       apple      1        2
    2       BBB                2       orange     1        1      
--------------------       -------------------------------------

Dim query as String = "SELECT item_name, TableA.code_name 
                       FROM TableB
                       LEFT JOIN TableA
                       ON TableB.code_1 = TableA.code_id
                       WHERE item_name = apple"

Result
-----------------------
item_name  code_name
-----------------------
 apple        AAA
-----------------------

以上查询只会返回一个代码名称，如果我想检索code_1和code_2？

Result that I want
----------------------------
 item_name  code_1   code_2
----------------------------
  apple      AAA      BBB
----------------------------

Answer 1

SELECT S1.item_name, U.code_name
    FROM tableA U
    JOIN tableB S1 ON (S1.code_1 = U.code_id)
    JOIN tableB S2 ON (S2.code_2 = U.code_id)
 WHERE item_name = apple"

Answer 2

您应该在SELECT中指定要反选的列

SELECT item_name, A.code_name, B.code_name FROM TableB WHERE item_name = 'apple'
LEFT JOIN TableA A ON TableB.code_1 = TableA.code_id 
LEFT JOIN TableA B ON TableB.code_2 = TableA.code_id