What is Foreign key in a table
Foreign key is a column in one table which is primary key on another table. Foreign key and Primary key is used to define relationship between two tables in relational database. For example in Employe and Department relationship, we have two tables Department(dept_id, dept_name) and Employee(emp_id, emp_name, dept_id). dept_id is primary key in Department table and foreign key in Employeetable. Though it’s not require that name of foreign key must be same with primary key, we have kept it same as per standard SQL best practices. Foreign key in a table enforce Referential Integrity constraint, which can be used to implement business rules e.g. referential integrity can stop you from creating an Employee with a non existent department. This kind of check maintains integrity of data in a relationship. As discussed in our post What is referential integrity in MySQL database, we have seen that it's implemented as foreign key constraint and can allow CASCADE UPDATE and DELETE. These referential action delete or update matching column in child table ( foreign key table) when corresponding row from parent table (primary key table ) is deleted or updated to maintain integrity of data.Difference between Primary key and Foreign key in SQL
Here are some important difference between primary and foreign keys in a table which is worth remembering both on SQL interview point of view and knowledge point of view.
1) Name of foreign key can be different than the name of primary key it represent in other table. For example in ourEmployee and Department relationship, Primary key in Department table is dept_id and we have used same name in Employee table to create foreign key. It could have been different e.g. departmentId or departmentID t etc.
2) Another difference between primary and foreign key is that unlike primary key, foreign key can be null e.g. in our example you can have an Employee record for which dept_id can be null, this shows that no corresponding record inDepartment table.
3) One more difference between primary key and foreign key is that foreign key can be duplicate opposite to primary key which is always unique.
4) By using foreign key constraints, we can introduce referential integrity in multiple table relationship in SQL. Referential integrity guarantees data integrity, see benefits of Referential Integrity in SQL to know more.
5) Foreign key mostly work as link between two table when we join tables using INNER JOIN and OUTER JOIN. For example when we INNER JOIN both Employee with Department table, we can use dept_id as joining column. SeeHow to join three tables in SQL for more details.
6) Table on which a column is declared as primary key is known as parent table in relationship and foreign keytable is known as child table in relationship. For example in Employee and Department relationship, Department is parent table because dept_id is primary key there and Employee is child table because dept_id is foreign key in this table.
Primary key and Foreign key Example in SQL
One of the best example to understand Primary key and Foreign key in a table is Employee and Department relationship or Customer and Order relationship. You can create Order and Customer table in MySQL as following to create primary and foreign keys :
CREATE TABLE Customer (cust_id INT NOT NULL,
cust_name VARCHAR(256),
PRIMARY KEY (cust_id)) ENGINE=INNODB;
CREATE TABLE ORDER (order_id INT NOT NULL,
amount INT NOT NULL,
cust_id INT,
FOREIGN KEY (cust_id) REFERENCES Customer(cust_id)
ON DELETE CASCADE) ENGINE=INNODB;
cust_name VARCHAR(256),
PRIMARY KEY (cust_id)) ENGINE=INNODB;
CREATE TABLE ORDER (order_id INT NOT NULL,
amount INT NOT NULL,
cust_id INT,
FOREIGN KEY (cust_id) REFERENCES Customer(cust_id)
ON DELETE CASCADE) ENGINE=INNODB;
Now cust_id is primary key in Customer table and foreign key in Order table. If we try to insert an Order for whichcust_id is something which is invalid in Customer table, MySQL database will reject such INSERT or UPDATE. This is one of the benefit of using Referential Integrity. It also allow to CASCADE UPDATE and DELETE operation which first delete or update a row in parent table e.g. Customer and then delete or update all matching rows in child table e.g. Order table.
That's all on what is foreign key in a table and difference between primary and foreign key in SQL. I suggest to create some table by yourself and try to test foreign key constraint by violating it and see how database e.g. Oracle, MySQL or SQL Server behaves. To understand more try ON DELETE CASCADE and ON DELETE UPDATE to see how database maintains foreign key constraint
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