SQL Select 声明：一个 Child - 两个 Parents （两个不同的FK）

Question

我遇到的情况最好通过以下场景来解释：

其中一条 Child 记录有两个 Parent（母亲和父亲）

如果您稍后阅读此post考虑应用self-referencing tables场景的选项，则没有必要，没有祖父或孙子。

关于parent是怎么定义的：

CREATE TABLE IF NOT EXISTS parent(
  code varchar(3),
  name varchar(10),
  PRIMARY KEY(code)
)ENGINE=INNODB;

记录插入方式：

INSERT INTO parent(code,name) VALUES('001','Mary');
INSERT INTO parent(code,name) VALUES('002','Joseph');
INSERT INTO parent(code,name) VALUES('003','Adan');
INSERT INTO parent(code,name) VALUES('004','Eva');
INSERT INTO parent(code,name) VALUES('005','Ana');
INSERT INTO parent(code,name) VALUES('006','Elcana');

并且 select 查询按预期工作：

mysql> select * from parent;
+------+--------+
| code | name   |
+------+--------+
| 001  | Mary   |
| 002  | Joseph |
| 003  | Adan   |
| 004  | Eva    |
| 005  | Ana    |
| 006  | Elcana |
+------+--------+
6 rows in set (0.01 sec)

关于child是怎么定义的：

CREATE TABLE IF NOT EXISTS child(
  code varchar(3),
  name varchar(10),
  PRIMARY KEY(code),
  mother_code varchar(3),
  father_code varchar(3),
  FOREIGN KEY fk_mother_code(mother_code) REFERENCES parent(code),
  FOREIGN KEY fk_father_code(father_code) REFERENCES parent(code)
)ENGINE=INNODB;

观察： 从上方观察 Child 期望两个 PK 来自 Parent（假设必须不同）通过两个 FKs.

记录插入方式：

INSERT INTO child(code, name, mother_code, father_code) VALUES('001','Jesus', '001', '002');
INSERT INTO child(code, name, mother_code, father_code) VALUES('002','Cain', '003', '004');
INSERT INTO child(code, name, mother_code, father_code) VALUES('003','Abel', '003', '004');
INSERT INTO child(code, name, mother_code, father_code) VALUES('004','Set', '003', '004');
INSERT INTO child(code, name, mother_code, father_code) VALUES('005','Samuel', '005', '006');

并且 select 查询按预期工作：

mysql> select * from child;
+------+--------+-------------+-------------+
| code | name   | mother_code | father_code |
+------+--------+-------------+-------------+
| 001  | Jesus  | 001         | 002         |
| 002  | Cain   | 003         | 004         |
| 003  | Abel   | 003         | 004         |
| 004  | Set    | 003         | 004         |
| 005  | Samuel | 005         | 006         |
+------+--------+-------------+-------------+
5 rows in set (0.00 sec)

目标是获得以下内容：

+------+--------+------+-------+-------------+-------------+
| code | name   | code | name  | mother_code | father_code |
+------+--------+------+-------+-------------+-------------+
| 001  | Mary   | 001  | Jesus | 001         | 002         |
| 002  | Joseph | 001  | Jesus | 001         | 002         |
+------+--------+------+-------+-------------+-------------+

我试过以下方法：

SELECT p.*, c.* FROM parent p,
                     child c,
                     (SELECT pm.code AS m_code FROM parent pm) AS m,
                     (SELECT pf.code AS f_code FROM parent pf) AS f
                WHERE
                     m.m_code='001' AND
                     f.f_code='002' AND
                     c.mother_code=m.m_code AND
                     c.father_code=f.f_code AND
                     c.mother_code='001' AND
                     c.father_code='002' AND
                     c.code='001';

where 子句看起来多余，这是因为我试图获得所需的结果，因此它包含尝试编写正确的查询。

但总是returns:

+------+--------+------+-------+-------------+-------------+
| code | name   | code | name  | mother_code | father_code |
+------+--------+------+-------+-------------+-------------+
| 001  | Mary   | 001  | Jesus | 001         | 002         |
| 002  | Joseph | 001  | Jesus | 001         | 002         |
| 003  | Adan   | 001  | Jesus | 001         | 002         |
| 004  | Eva    | 001  | Jesus | 001         | 002         |
| 005  | Ana    | 001  | Jesus | 001         | 002         |
| 006  | Elcana | 001  | Jesus | 001         | 002         |
+------+--------+------+-------+-------------+-------------+
6 rows in set (0.00 sec)

那么正确的句子是什么？

Answer 1

您是要找两个 join 吗？

select c.*, pm.name as mother_name, pf.name as father_name
from child c join
     parent pm
     on c.mother_code = pm.code join
     parent pf
     on c.father_code = pf.code;

您可以添加一个 where 子句来将其过滤到特定的 children:

where c.code in ('001', '002')

Answer 2

根据您的预期结果，我认为这就是您需要的：

select
  p.code, p.name,
  c.code, c.name,
  c.mother_code, c.father_code
from parent p 
inner join child c 
on c.mother_code = p.code or c.father_code = p.code

您可以使用 WHERE 子句添加您需要的任何条件。
见 demo.
结果：

| code | name   | code | name   | mother_code | father_code |
| ---- | ------ | ---- | ------ | ----------- | ----------- |
| 001  | Mary   | 001  | Jesus  | 001         | 002         |
| 002  | Joseph | 001  | Jesus  | 001         | 002         |
| 003  | Adan   | 002  | Cain   | 003         | 004         |
| 004  | Eva    | 002  | Cain   | 003         | 004         |
| 003  | Adan   | 003  | Abel   | 003         | 004         |
| 004  | Eva    | 003  | Abel   | 003         | 004         |
| 003  | Adan   | 004  | Set    | 003         | 004         |
| 004  | Eva    | 004  | Set    | 003         | 004         |
| 005  | Ana    | 005  | Samuel | 005         | 006         |
| 006  | Elcana | 005  | Samuel | 005         | 006         |

Answer 3

如果依赖于 child 以获取 parent，请使用 or 加入。

SELECT p.Code, p.[Name], c.Code, p.[Name], c.Mother_Code, c.Father_Code
FROM Parent p JOIN Child c ON c.Mother_Code = p.Code OR c.Father_Code = p.Code
WHERE c.name = 'Jesus'

如果依赖是找一个parent的children，那么改一下WHERE语句就可以了

SELECT p.Code, p.[Name], c.Code, p.[Name], c.Mother_Code, c.Father_Code
FROM Parent p JOIN Child c ON c.Mother_Code = p.Code OR c.Father_Code = p.Code
WHERE p.name IN ('Mary', 'Joseph')

SQL Select 声明：一个 Child - 两个 Parents （两个不同的FK）

SQL Select Statement: One Child - Two Parents (two different FK)

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