Question

What is a foreign key and a primary key and how do they relate to each other?

Why are foreign keys and the referential rule important in representing relationships within a database?

Why does declaring a primary key represent a constraint on the values possible in a relation

Answer 1

Candidate Key:

A candidate key is an attribute or set of attributes that uniquely and minimally identify an entity instance in an entity set or record in a table. In a single table, there may be more than one candidate keys.

Primary Key:

The primary key is one of the selected candidate keys to identify the record uniquely.  

Foreign Key:

A foreign key is a single column or group of columns that establish a connection between two tables and it works as a primary key in another table. A table can have more than one foreign key.

The primary key of the table works as a foreign key for another table.

Referential Integrity Rule:

Referential integrity is a constraint on the system that is applied between two tables and it states that non-null values of a foreign key are valid if they appear in the parent key. In other words, we can say that every foreign key value must be from the primary key of another table or it may be null.

These are the constraints on our system which avoid the entry of incorrect data.

The primary key is a single column or group of the column that can identify the record uniquely and a table can have only one primary key.

Primary key constraints are:

• In the primary key field, duplicate values are not allowed.
• The primary key can't be null.

So, if primary key is declared then these constraints must be followed while we are inserting or updating the table.

The keys are nothing but these are constraints on our system.