MySQL中的多列外鍵？

Question

我有一個主鍵由兩列組成的表（product_id，attribute_id）。我有另一個表需要引用此表。如何在另一個表中使外鍵將其鏈接到表中具有兩個主鍵的行上？

Answer 1

像這樣的東西應該做到這一點：

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的需要外鍵進行索引，因此在引用列的索引
• 使用約束語法可以命名約束，如果需要，可以在稍後的時間更容易地進行修改和刪除。
• InnoDB強制外鍵，MyISAM不強制。 （語法分析，但是忽略）

Answer 2

表中只能有一個主鍵。可以由多個字段組成的事實不會增加主鍵的數量，但仍然有一個。

由於PK對的一部分不是唯一的，所以顯然必須創建引用兩個字段的外鍵：REFERENCES t1（f1，f2）。

Answer 3

如果我們想的邏輯外鍵一些這樣

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') 

選信號：

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)