All the keys which are not primary key are called an alternate key. It is a candidate key which is currently not the primary key. However, A table may have single or multiple choices for the primary key.
Lets take an example to understand the alternate key concept. Here we have a table Employee, this table has three attributes: Emp_Id, Emp_Number & Emp_Name.
| emp_Id | Emp_Number | Emp_Name |
|---|---|---|
| E01 | 1214 | Prayag |
| E22 | 2272 | Pankaj |
| E23 | 3261 | Rakesh |
| E44 | 4825 | Hazra |
{emp_Id}
and
{Emp_Number}
DBA (Database administrator) can choose any of the above key as primary key. Lets say Emp_Id is chosen as primary key.
Since we have selected Emp_Id as primary key, the remaining key Emp_Number would be called alternative or secondary key.