A foreign key is a column (or combination of columns) in a table whose values must match values of a column in some other table. The purpose of foreign keys is to enforce referential integrity, which essentially says that if column value A refers to column value B, then column value B must exist.


How do you define a foreign key? A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It acts as a cross-reference between tables because it references the primary key of another table, thereby establishing a link between them.

what is a foreign key example?

A foreign key is a column (or columns) that references a column (most often the primary key) of another table. For example, say we have two tables, a CUSTOMER table that includes all customer data, and an ORDERS table that includes all customer orders.

What is foreign key in DBMS? Definition: Foreign keys are the columns of a table that points to the primary key of another table. They act as a cross-reference between tables. For example: In the below example the Stu_Id column in Course_enrollment table is a foreign key as it points to the primary key of the Student table.

how do I disable foreign key check?

In your table, go to SQL tab. After you edit the SQL command that you want to run, there is a check box next to GO , named 'Enable foreign key checks' . Uncheck this check box and run your SQL .

Is foreign key unique? Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). That is all an FK is by definition.

what does a foreign key constraint do?

The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables. The FOREIGN KEY constraint also prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the table it points to.

What is foreign key relationship? A primary key-foreign key relationship defines a one-to-many relationship between two tables in a relational database. A foreign key is a column or a set of columns in one table that references the primary key columns in another table.

How do you know if a table has a foreign key?

To see foreign key relationships of a table: SELECT TABLE_NAME, COLUMN_NAME, CONSTRAINT_NAME, REFERENCED_TABLE_NAME, REFERENCED_COLUMN_NAME FROM INFORMATION_SCHEMA. KEY_COLUMN_USAGE WHERE REFERENCED_TABLE_SCHEMA = 'db_name' AND REFERENCED_TABLE_NAME = 'table_name';

Where do you put a foreign key? REFERENCES constraint to the table for it to be a foreign key. When you join the two tables together, the primary key of the parent table will be set equal to the foreign key of the child table. Whichever one is not the primary key is the foreign key. In one-to-many relationships, the FK goes on the "many" side.

Can we create foreign key without primary key?

Yes. We can create a Foreign key without having Primary key in the parent table. Most of us are in a misconception that we can not create a Foreign key without primary key in the parent table. That unique id can be primary key or unique key.

Can foreign key be deleted?

A foreign key with cascade delete means that if a record in the parent table is deleted, then the corresponding records in the child table will automatically be deleted. A foreign key with cascade delete can be created using either a CREATE TABLE statement or an ALTER TABLE statement.

Are foreign keys necessary?

24 Answers. Foreign keys help enforce referential integrity at the data level. They also improve performance because they're normally indexed by default. Foreign keys can also help the programmer write less code using things like ON DELETE CASCADE .

Is foreign key a primary key?

Foreign key is a field in the table that is primary key in another table. Primary Key can't accept null values. Foreign key can accept multiple null value. By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.

Can foreign key contain null?

A foreign key makes its table dependent on another table called a parent table. A foreign key containing null values cannot match the values of a parent key, since a parent key by definition can have no null values. However, a null foreign key value is always valid, regardless of the value of any of its non-null parts.

What is foreign key violation?

Occurs when an update or delete on a primary key table would violate a foreign key constraint. Occurs when an insert or update on a foreign key table is performed without a matching value in the primary key table.

Can foreign key have duplicate values?

Unlike primary keys, foreign keys can contain duplicate values. Also, it is OK for them contain NULL values. Indexes aren't automatically created for foreign keys; however, as a DBA, you can define them. A table is allowed to contain more than one foreign key.

How do I remove a foreign key?

To delete a foreign key constraint In Object Explorer, expand the table with the constraint and then expand Keys. Right-click the constraint and then click Delete. In the Delete Object dialog box, click OK.