Explain the distinctions among the terms primary key, candidate key, and su-perkey.

1. Primary Key: A primary key is a special relational database table column (or combination of columns) designated to uniquely identify all table records. It has the unique feature of not having any duplicate values. Once a primary key is defined, you cannot have null or duplicate values. This is the main key used to identify unique rows in a table.

2. Candidate Key: A candidate key is a column, or set of columns, in a table that can uniquely identify any database record without referring to any other data. Each table may have one or more candidate keys, but one candidate key is unique, and it is selected as the primary key. The remaining keys are known as secondary or alternate keys.

3. Super Key: A super key is a group of single or multiple keys which identifies rows in a table. It is a set of attributes, one or more, that can uniquely identify a record in a database table. It can be a single key or a combination of keys. It is a superset of candidate keys.

In summary, a primary key is a special kind of candidate key that is chosen by the database designer to uniquely identify the entity set. A candidate key is a minimal superkey, i.e., a superkey from which we cannot remove any attributes and still have the uniqueness property. A superkey is a set of one or more columns to which, when we add other columns, we still have the uniqueness property.