Question

我有一个表，其主键由两列组成（product_id，attribute_id）。我有另一个表需要引用此表。如何在另一个表中创建一个外键，使用两个主键将其链接到表中的一行？

Answer 1

这样的事情应该这样做：

CREATE TABLE MyReferencingTable AS (
   [COLUMN DEFINITIONS]
   refcol1 INT NOT NULL,
   rofcol2 INT NOT NULL,
   CONSTRAINT fk_mrt_ot FOREIGN KEY (refcol1, refcol2)
                        REFERENCES OtherTable(col1, col2)
) ENGINE=InnoDB;

MySQL要求对外键进行索引，因此引用列上的索引
使用约束语法可以命名约束，如果需要，可以在以后更容易更改和删除。
InnoDB强制执行外键，MyISAM则不执行。（语法被解析但被忽略）

Answer 2

表上只能有一个主键。事实上可以包含多个字段，但不会增加主键的数量，还有一个。

由于PK对的一部分不是唯一的，你显然必须创建一个引用两个字段的外键：REFERENCES t1（f1，f2）。

Answer 3

如果我们想要像这样的外键的逻辑

FOREIGN KEY COmments(issue_id)
REFERENCES Bugs(issue_id) OR FeatureRequests(issue_id)

示例：

CREATE TABLE Issues (
issue_id int PRIMARY KEY,
status VARCHAR(20)

);




CREATE TABLE Comments (
comment_id int PRIMARY KEY,
issue_type VARCHAR(20), -- "Bugs" or "FeatureRequests"
issue_id BIGINT UNSIGNED NOT NULL,
comment TEXT
);



CREATE TABLE Bugs (
issue_id int PRIMARY KEY,
severity VARCHAR(20),
FOREIGN KEY (issue_id) REFERENCES Issues(issue_id)
);
CREATE TABLE FeatureRequests (
issue_id int PRIMARY KEY,
sponsor VARCHAR(50),
FOREIGN KEY (issue_id) REFERENCES Issues(issue_id)
);





INSERT INTO Issues VALUES(1,'ON'),(2,'ON'),(3,'OFF'),(6,'OFF'),(8,'ON');

INSERT INTO Comments VALUES(1,'Bugs',1,'A'),(2,'Bugs',3,'B'),(3,'Bugs',1,'C'),(4,'Bugs',3,'D'),(5 ,'FeatureRequests',8,'L'),
(6,'FeatureRequests',6,'W'),(7,'FeatureRequests',1,'ZX');



INSERT INTO Bugs VALUES(1,'severity_1'),(3,'severity_for_3');


INSERT INTO FeatureRequests VALUES(2,'sponsor_2_'),(8,'sponsor_for_8'),(1,'sponsor_for_1')

选择：

MariaDB [test]> SELECT * FROM Comments JOIN FeatureRequests  ON Comments.issue_i
d = FeatureRequests.issue_id AND Comments.issue_type= 'FeatureRequests';


MariaDB [test]> SELECT * FROM Comments JOIN Bugs  ON Comments.issue_id = Bugs.is
sue_id AND Comments.issue_type= 'Bugs';
+------------+------------+----------+---------+----------+----------------+
| comment_id | issue_type | issue_id | comment | issue_id | severity       |
+------------+------------+----------+---------+----------+----------------+
|          1 | Bugs       |        1 | A       |        1 | severity_1     |
|          2 | Bugs       |        3 | B       |        3 | severity_for_3 |
|          3 | Bugs       |        1 | C       |        1 | severity_1     |
|          4 | Bugs       |        3 | D       |        3 | severity_for_3 |
+------------+------------+----------+---------+----------+----------------+
4 rows in set (0.00 sec)

MySQL中的多列外键？

3 个答案: