增加外键

建表的时候增加

CREATE TABLE Members (

Member_ID INT NOT NULL PRIMARY KEY AUTO_INCREMENT,

First_Name VARCHAR(30) NOT NULL,

Last_Name VARCHAR(30) NOT NULL,

Birthday DATE NOT NULL,

Gender ENUM('M', 'F') DEFAULT 'F'

);

CREATE TABLE Email (

Email VARCHAR(50),

Member_ID INT NOT NULL,

PRIMARY KEY (Email, Member_ID),

CONSTRAINT FOREIGN KEY (Member_ID) REFERENCES Members(Member_ID)

ON DELETE  RESTRICT  ON UPDATE CASCADE

);

建表后增加外键

ALTER TABLE Email add CONSTRAINT FOREIGN KEY(Member_ID) REFERENCES Members(Member_ID)