如果引用存在,则连接表

Question

我有一个包含4个表的PostgreSQL数据库:

表A.

---------------------------
| ID | B_ID | C_ID | D_ID |
---------------------------
| 1  |  1   | NULL | NULL |
---------------------------
| 2  | NULL |   1  | NULL |
---------------------------
| 3  |  2   |   2  |   1  |
---------------------------
| 4  | NULL | NULL |   2  |
---------------------------

表B.

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| ID | DATA |
-------------
| 1  | 123  |
-------------
| 2  | 456  |
-------------

表C.

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| ID | DATA |
-------------
| 1  | 789  |
-------------
| 2  | 102  |
-------------

表D.

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| ID | DATA |
-------------
| 1  | 654  |
-------------
| 2  | 321  |
-------------

我正在尝试检索已加入表B中的数据和表C中的数据的结果集,但前提是其中一个booth ID不为null.

SELECT "Table_A"."ID", "Table_A"."ID_B", "Table_A"."ID_C", "Table_A"."ID_D", "Table_B"."DATA", "Table_C"."DATA"
    FROM "Table_A"
        LEFT JOIN "Table_B" on "Table_A"."ID_B" = "Table_B"."ID"
        LEFT JOIN "Table_C" on "Table_A"."ID_C" = "Table_C"."ID"
    WHERE "Table_A"."ID_B" IS NOT NULL OR "Table_A"."ID_C" IS NOT NULL;

这是推荐还是我应该在多个查询中更好地拆分它？

有没有办法在这些表之间进行内部联接？

我期望的结果是:

-------------------------------------------------
| ID | ID_B | ID_C | ID_D | DATA (B) | DATA (C) |
-------------------------------------------------
| 1  |   1  | NULL | NULL |   123    |   NULL   |
-------------------------------------------------
| 2  | NULL |  1   | NULL |   NULL   |   789    |
-------------------------------------------------
| 3  |   2  |  2   | NULL |   456    |   102    |
-------------------------------------------------

编辑: ID_B,ID_C,ID_D有外键的表table_b,table_c,table_d

Answer 1

该WHERE "Table_A"."ID_B" IS NOT NULL OR "Table_A"."ID_C" IS NOT NULL;可通过B和C表上的相应条款取代:WHERE "Table_B"."ID" IS NOT NULL OR "Table_C"."ID" IS NOT NULL;.如果table_a.id_b和table_a.id_c不是B和C表的FK,这也可以工作.否则,具有{5,5,5,5}的table_a行将从B和C表中检索两个NULL行.

SELECT ta."ID" AS a_id
        , ta."ID_B" AS b_id
        , ta."ID_C" AS c_id
        , ta."ID_D" AS d_id
        , tb."DATA" AS bdata
        , tc."DATA" AS cdata
FROM "Table_a" ta
LEFT JOIN "Table_B" tb on ta."ID_B" = tb."ID"
LEFT JOIN "Table_C" tc on ta."ID_C" = tc."ID"
WHERE tb."ID" IS NOT NULL OR tc."ID" IS NOT NULL
        ;