Question

我有以下表格

Table A             Table B             Table C 
ColumnA ColumnB     ColumnA ColumnB     ColumnA ColumnB
1       A                 2 X             X     Value1
2       B                 3 Y             Y     Value2
3       C                 5 Z             Z     Value3
4       D   
5       E

所需的结果是

Column1 Column2   Column3

 1                A
 2       Value1   B
 3       Value2   C
 4                D
 5       Value3   E

我一直在玩左外连接。但仍然没有接近我正在寻找的结果。任何帮助表示赞赏。

Answer 1

您需要使用LEFT JOIN两次：

CREATE table tablea (
  columna NUMBER,
  columnb VARCHAR2(1)
);

CREATE table tableb (
  columna NUMBER,
  columnb VARCHAR2(1)
);

CREATE table tablec (
  columna VARCHAR2(1),
  columnb VARCHAR2(10)
);

INSERT INTO tablea VALUES (1, 'A');
INSERT INTO tablea VALUES (2, 'B');
INSERT INTO tablea VALUES (3, 'C');
INSERT INTO tablea VALUES (4, 'D');
INSERT INTO tablea VALUES (5, 'E');

INSERT INTO tableb VALUES (2, 'X');
INSERT INTO tableb VALUES (3, 'Y');
INSERT INTO tableb VALUES (5, 'Z');

INSERT INTO tablec VALUES ('X', 'Value1');
INSERT INTO tablec VALUES ('Y', 'Value2');
INSERT INTO tablec VALUES ('Z', 'Value3');

COMMIT;

SELECT ta.columna, tc.columnb, ta.columnb
  FROM tablea ta
    LEFT JOIN tableb tb ON (ta.columna = tb.columna)
    LEFT JOIN tablec tc ON (tc.columna = tb.columnb)
ORDER BY 1    
;

输出：

   COLUMNA COLUMNB    COLUMNB
---------- ---------- -------
         1            A       
         2 Value1     B       
         3 Value2     C       
         4            D       
         5 Value3     E

SQLFiddle demo

Oracle加入以获得理想的结果

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