我有一个表,它有两列作为主键 (product_id, attribute_id)。我有另一个需要引用该表的表。如何在另一个表中创建外键以将其链接到具有两个主键的表中的行?
我有一个表,它有两列作为主键 (product_id, attribute_id)。我有另一个需要引用该表的表。如何在另一个表中创建外键以将其链接到具有两个主键的表中的行?
像这样做应该就可以了:
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;
一张表只能有一个主键。即使主键由多个字段组成,也不会增加主键的数量,仍然只有一个。
由于主键对中的某一部分不是唯一的,因此您显然还需要创建一个外键,将两个字段作为参考:REFERENCES t1(f1,f2)。
FOREIGN KEY COmments(issue_id)
REFERENCES Bugs(issue_id) OR FeatureRequests(issue_id)
例子:
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')
选择查询:
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要求外键被索引,因此在引用列上建立索引
- 这不应该是被引用的列
吗? - Vérace