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This article mainly explains "the introduction and example usage of foreign key in mysql". The content of the explanation in the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "the introduction and example usage of foreign key in mysql".

1. To constrain a field in a table, it must be a value that exists in a field in another table.

2. In another table, this column is not necessarily a primary key, but it must be a unique index, otherwise creation will fail.

Example

Mysql > create table orders (id int (11) primary key, userId int (11), FOREIGN KEY (userId) REFERENCES user (id)); Query OK, 0 rows affected, 2 warnings (0.06 sec) mysql > insert orders values (1Magol 3); ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`t`.`orders`, CONSTRAINT `orders_ibfk_ 1` FOREIGN KEY (`userId`) REFERENCES `user` (`id`) mysql > insert orders values (1Pol 1) Query OK, 1 row affected (0.01sec) Thank you for your reading. The above is the content of "introduction and example usage of foreign key in mysql". After the study of this article, I believe you have a deeper understanding of the introduction and example usage of foreign key in mysql, and the specific use needs to be verified by practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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