How to create foreign key constraints in mysql

Mysql method to establish foreign key constraints: directly execute the [CREATE TABLE stu(sid INT PRIMARY KEY,NAME VARCHAR(50) NOT NULL);] statement.

Add foreign key constraints

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CREATE TABLE stu( sid INT PRIMARY KEY, NAME VARCHAR(50) NOT NULL );

Add foreign key constraint method one

CREATE TABLE score1( score DOUBLE, sid INT, CONSTRAINT fk_stu_score1_sid FOREIGN KEY(sid) REFERENCES stu(sid) );

Add foreign key constraint method two (if the table already exists, you can use this method)

CREATE TABLE score1( score DOUBLE, sid INT ); ALTER TABLE score1 ADD CONSTRAINT fk_sid FOREIGN KEY(sid) REFERENCES stu(sid)

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