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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