如何將兩個主鍵用於一個外鍵？

Question

mysql> CREATE TABLE guests (
    -> id int(11) NOT NULL AUTO_INCREMENT, 
    -> first_name varchar(55) DEFAULT NULL, 
    -> last_name varchar(55) DEFAULT NULL, 
    -> email varchar(100) DEFAULT NULL, 
    -> role_type int(4) DEFAULT 4, 
    -> start_date date, 
    -> expiry_date date, 
    -> reason blob, 
    -> added_by int(4) NOT NULL REFERENCES super_admins(id), 
    -> password_digest varchar(255) DEFAULT NULL, 
    -> remember_token date DEFAULT NULL, 
    -> primary key(id), 
    -> CONSTRAINT fk_guests FOREIGN KEY(added_by) REFERENCES super_admins(id), 
    -> CONSTRAINT fk_guests1 FOREIGN KEY (added_by) REFERENCES admin(id) 
    ->); 

Answer 1

它被稱爲多態關係（關聯）。你不能有一個列（added_by你的情況），同時引用兩個父表。但是你可以做什麼才能使用外鍵約束是有兩個可爲空的列added_by_super_admin和added_by_admin，其中只有一列將保存每個記錄的值。

CREATE TABLE guests 
(
    id INT(11) NOT NULL AUTO_INCREMENT, 
    first_name VARCHAR(55) DEFAULT NULL, 
    last_name VARCHAR(55) DEFAULT NULL, 
    email VARCHAR(100) DEFAULT NULL, 
    role_type INT(4) DEFAULT 4, 
    start_date DATE, 
    expiry_date DATE, 
    reason blob, 
    added_by_super_admin INT(4) NULL, 
    added_by_admin INT(4) NULL, 
    password_digest VARCHAR(255) DEFAULT NULL, 
    remember_token DATE DEFAULT NULL, 
    PRIMARY KEY(id), 
    CONSTRAINT fk_guests FOREIGN KEY (added_by_super_admin) REFERENCES super_admins(id), 
    CONSTRAINT fk_guests1 FOREIGN KEY (added_by_admin) REFERENCES admin(id) 
); 

然後在查詢中，你可以做

SELECT ... 
     COALESCE(s.id, a.id) added_by_id, 
     COALESCE(s.name, a.name) added_by_name, 
     ... 
    FROM guests g LEFT JOIN super_admins s 
    ON g.added_by_super_admin = s.id LEFT JOIN admin 
    ON g.added_by_admin = a.id