What is Foreign Key?

A foreign key in one table points to a primary key in another table. Foreign keys prevent actions that would leave rows with foreign key values when there are no primary keys with that value. The foreign key constraints are used to enforce referential integrity. Foreign keys are both a method of ensuring data integrity and a manifestation of the relationship between tables.

from Tulika:
Definition: A foreign key is a field in a relational table that matches the primary key column of another table. The foreign key can be used to cross-reference tables.
In the context of relational databases, a foreign key is a referential constraint between two tables. The foreign key identifies a column or a set of columns in one (referencing) table that refers to set of columns in another (referenced) table. The columns in the referencing table must be the primary key or other candidate key in the referenced table. The values in one row of the referencing columns must occur in a single row in the referenced table. Thus, a row in the referencing table cannot contain values that don't exist in the referenced table (except potentially NULL). This way references can be made to link information together and it is an essential part of database normalization. Multiple rows in the referencing table may refer to the same row in the referenced table. Most of the time, it reflects the one (master table, or referenced table) to many (child table, or referencing table) relationship.
The referencing and referenced table may be the same table, i.e. the foreign key refers back to the same table. Such a foreign key is known in SQL:2003 as a self-referencing or recursive foreign key.
Because the Database Management System enforces referential constraints, it must ensure data integrity if rows in a referenced table are to be deleted (or updated). If dependent rows in referencing tables still exist, those references have to be considered. SQL:2003 specifies 5 different referential actions that shall take place in such occurrences:

CASCADE
RESTRICT
NO ACTION
SET NULL
SET DEFAULT

 
 
 
 
 
 
 
 


from Shivani:
A foreign key (FK) is a column or combination of columns used to establish and enforce a link between the data in two tables. A link is created between two tables by adding the column or columns that hold one table's primary key values to the other table. This column becomes a foreign key in the second table.
You can create a foreign key by defining a FOREIGN KEY constraint when you create or alter a table.
For example, the titles table(Title_ID as Primary Key) has a link to the publishers table(Pub_ID as primary Key) because there is a logical relationship between books and publishers. The pub_id column in the titles table matches the primary key column of the publishers table. The pub_id column in the titles table is the foreign key to the publishers table.

A FOREIGN KEY constraint does not have to be linked only to a PRIMARY KEY constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table. A FOREIGN KEY constraint can contain null values; however, if any column of a composite FOREIGN KEY constraint contains null values, then verification of the FOREIGN KEY constraint will be skipped.