MySQL (and its fork MariaDB) are Relational Database Management Systems ( RDBMS) – Database Systems which hold data in tables which can be related to each other. In this tutorial, we have introduced you to the foreign key concept and shown you how to create foreign key using SQL FOREIGN KEY constraint.In this guide, we cover foreign key constraints in MySQL and MariaDB, along with a number of useful examples. If you are using MySQL, you can use a cleaner syntax as follows: ALTER TABLE table_nameĭROP FOREIGN KEY fk_name Code language: SQL (Structured Query Language) ( sql )įor example, to remove the fk_project foreign key constraint, you use the following statement: ALTER TABLE project_milestonesĭROP CONSTRAINT fk_project Code language: SQL (Structured Query Language) ( sql ) To remove a foreign key constraint, you also use the ALTER TABLE statement as follows: ALTER TABLE table_nameĭROP CONSTRAINT fk_name Code language: SQL (Structured Query Language) ( sql ) REFERENCES projects(project_id) Code language: SQL (Structured Query Language) ( sql ) Removing foreign key constraints To do so, you use the following ALTER TABLE statement: ALTER TABLE project_milestonesĪDD CONSTRAINT fk_project FOREIGN KEY(project_id) Suppose the project_milestones already exists without any predefined foreign key and you want to define a FOREIGN KEY constraint for the project_id column. REFERENCES table_2(pk_key_column) Code language: SQL (Structured Query Language) ( sql ) ALTER TABLE table_1ĪDD CONSTRAINT fk_name FOREIGN KEY (fk_key_column) To add a FOREIGN KEY constraint to existing table, you use the ALTER TABLE statement. Adding FOREIGN KEY contraints to existing tables You can assign a name to a FOREIGN KEY constraint as follows: CREATE TABLE project_milestones (ĬONSTRAINT fk_project FOREIGN KEY (project_id)įk_project is the name of the FOREIGN KEY constraint. REFERENCES projects (project_id) Code language: SQL (Structured Query Language) ( sql ) The FOREIGN KEY clause promotes the project_id of the project_milestones table to become the foreign key that is referenced to the project_id of the projects table. You can create the FOREIGN KEY constraint when you create the table as follows: CREATE TABLE project_milestones ( The solution is to add an SQL FOREIGN KEY constraint to the project_milestones table to enforce the relationship between the projects and project_milestones tables. This causes the application not to work properly. Or user may delete a row in the projects table, leaving orphaned rows in the project_milestones table. Unfortunately, users may edit the database using client tool or if there is a bug in the application, a row might be added to the project_milestones table that does not correspond to any row in the projects table. In other words, a milestone cannot exist without a project. The application that uses these tables must ensure that for each row in the project_milestones table there exists the corresponding row in the projects table. ) Code language: SQL (Structured Query Language) ( sql )Įach project may have zero or more milestones while one milestone must belong to one and only one project. Milestone_id INT AUTO_INCREMENT PRIMARY KEY, Project_id INT AUTO_INCREMENT PRIMARY KEY, See the following projects and project_assignments tables: CREATE TABLE projects ( You use the FOREIGN KEY constraint to create a foreign key when you create or alter table. Let’s take a simple example to get a better understanding. The column (or columns) of the second table becomes the foreign key. In a foreign key reference, the primary key column (or columns) of the first table is referenced by the column (or columns) of the second table. Introduction to SQL foreign key constraintĪ foreign key is a column or a group of columns that enforces a link between the data in two tables. Summary: in this tutorial, you will learn about the SQL foreign key and how to create a FOREIGN KEY constraint to enforce the relationship between tables.
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