- Candidate Key is a key which provides the uniqueness of the column(s). It identifies each row of a table as unique. It can become the primary key of the table as well. Every tabular relationship will have atleast one candidate key.

- Alternate Key is a type of candidate key which is formed when there are more than one candidate key and one of them is a primary key then other keys will act as an alternate keys. Unique keys also termed as alternate keys which prevent incorrect data from entering the table.

- Composite Key is a special type of candidate key as it is formed by combining two or more columns. This gives assurance of uniqueness of data when the columns are joined together.