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Learn how to create, alter and drop FOREIGN KEY constraints in MySQL. A FOREIGN KEY is a field that refers to the PRIMARY KEY in another table and prevents invalid data.
Following is the basic syntax of defining a foreign key constraint in CREATE TABLE and ALTER TABLE constraint in MySQL. [CONSTRAINT constraint_name] FOREIGN KEY [foreign_key_name] (col_name, ...) REFERENCES parent_tbl_name (col_name,...) [ON DELETE reference_option] [ON UPDATE reference_option]
To add a foreign key (grade_id) to an existing table (users), follow the following steps: ALTER TABLE users ADD grade_id SMALLINT UNSIGNED NOT NULL DEFAULT 0; ALTER TABLE users ADD CONSTRAINT fk_grade_id FOREIGN KEY (grade_id) REFERENCES grades(id);
The essential syntax for a defining a foreign key constraint in a CREATE TABLE or ALTER TABLE statement includes the following: [CONSTRAINT [symbol]] FOREIGN KEY. [index_name] (col_name, ...) REFERENCES tbl_name (col_name,...) [ON DELETE reference_option] [ON UPDATE reference_option] reference_option:
Summary: in this tutorial, you will learn about MySQL foreign key and how to create, drop, and disable a foreign key constraint. Introduction to MySQL foreign key. A foreign key is a column or group of columns in a table that links to a column or group of columns in another table.
5.6.6 Using Foreign Keys. MySQL supports foreign keys, which permit cross-referencing related data across tables, and foreign key constraints, which help keep the related data consistent. A foreign key relationship involves a parent table that holds the initial column values, and a child table with column values that reference the parent column ...
SQL FOREIGN KEY on CREATE TABLE. The following SQL creates a FOREIGN KEY on the "PersonID" column when the "Orders" table is created: MySQL: CREATE TABLE Orders ( OrderID int NOT NULL, OrderNumber int NOT NULL, PersonID int, PRIMARY KEY (OrderID), FOREIGN KEY (PersonID) REFERENCES Persons (PersonID) ); SQL Server / Oracle / MS Access:
