As we know that database uses tables to organize information.
To maintain data integrity (that is data should be correct and in well formed)
we use concept of keys. There are five
types of keys in database which is as follows---
1)
Candidate key
2)
Primary Key
3)
Foreign Key
4)
Alternate Key
5)
Composite Key.
Candidate Key
Candidate keys are those keys which is candidate for primary
key of a table. In simple words we can understand that such type of keys which
full fill all the requirements of primary key which is not null and have unique
records is a candidate for primary key. So thus type of key is known as
candidate key.
Primary Key
Such type of candidate key which is chosen as a primary key
for table is known as primary key. Primary keys are used to identify tables.
There is only one primary key per table. In SQL Server when we create primary key
to any table then a clustered index is automatically created to that column.
Foreign Key
Foreign key are those keys which is used to define relationship
between two tables. When we want to implement relationship between two tables
then we use concept of foreign key. It is also known as referential integrity. We
can create more than one foreign key per table.
Alternate Key
If any table have more than one candidate key, then after choosing
primary key from those candidate key, rest of candidate keys are known as an
alternate key of that table. Like here we can take a very simple example to
understand the concept of alternate key. Suppose we have a table named Employee which has two columns EmpID
and EmpMail, both have not null attributes and unique value. So both columns
are treated as candidate key. Now we make EmpID as a primary key to that table
then EmpMail is known as alternate key.
Composite Key
When we create keys on more than one column then that key is
known as composite key. Like here we can take an example to understand this
feature. I have a table Student which has
two columns Sid and SrefNo and we make primary key on these two column. Then this
key is known as composite key.