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This article mainly introduces to you what methods can add mysql foreign keys, the content of the article is carefully selected and edited by the author, with a certain pertinence, for everyone's reference significance is still relatively great, the following with the author to understand what methods can add mysql foreign keys bar.

Mysql can add foreign keys directly after attribute values, at the end of SQL statements, using fk and adding foreign keys after table creation.

Four ways to add Foreign Keys to mysql

Create master table: class

CREATE TABLE class (cid INT PRIMARY KEY AUTO_INCREMENT,sname VARCHAR (15) NOT NULL) INSERT INTO class VALUES (Class 1, NULL,' Grade 6) INSERT INTO class VALUES (Class 2, Grade 6, NULL,')

First: add directly after the attribute value

CREATE TABLE student (sid INT AUTO_INCREMENT,sname VARCHAR (10) NOT NULL,s_cid INT REFERENCES class (cid), PRIMARY KEY (sid))

/ / add data for testing

INSERT INTO student VALUES (NULL,' King sledgehammer', 2) INSERT INTO student VALUES (NULL,' Hu Hansan', 3) / / the constraint does not take effect. The reason to be checked is INSERT INTO student VALUES (NULL,' shit', 4) / / unbelievable, once again tested.

This way, the test does not know why, actually added WTF? Baidu failed on the Internet, and the self-study group had no reply and put it on hold here for the time being.

The second kind: 2. Add at the end of the sql statement

CREATE TABLE student (sid INT AUTO_INCREMENT,sname VARCHAR (10), s_cid INT,PRIMARY KEY (sid), FOREIGN KEY (s_cid) REFERENCES class (cid))

/ / insert data to test

INSERT INTO student VALUES (NULL,' King sledgehammer', 2) INSERT INTO student VALUES (NULL,' Hu Hansan', 3) / constraint effective insert failed

Foreign key constraint takes effect

The third kind: the use of fk

CREATE TABLE student (sid INT AUTO_INCREMENT,sname VARCHAR (10) NOT NULL,s_cid INT, PRIMARY KEY (sid), CONSTRAINT fk_student_class FOREIGN KEY (s_cid) REFERENCES class (cid)); INSERT INTO student VALUES (NULL,' King sledgehammer', 2) INSERT INTO student VALUES (NULL,' Hu Hansan', 3) / constraint entry failure

Fourth: add foreign keys after creating the table

CREATE TABLE student (sid INT AUTO_INCREMENT,sname VARCHAR (10) NOT NULL,s_cid INT, PRIMARY KEY (sid))

/ / add a foreign key constraint:

ALTER TABLE student ADD FOREIGN KEY (s_cid) REFERENCES class (cid)

/ / Test

INSERT INTO student VALUES (NULL,' King sledgehammer', 2) INSERT INTO student VALUES (NULL,' Hu Hansan', 3) / constraint effective insert failed

After reading the above about what ways to add mysql foreign keys, many readers must have some understanding, if you need to get more industry knowledge and information, you can continue to pay attention to our industry information column.

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