Explain the terms primary key, candidate key, alternate key and secondary key. In the given table identify every key?

STUDENT(SID,Regno, Name, City)

Primary Key: The primary key of a relational table uniquely identify every record in the table. It can either be a general attribute (or set of attributes) which is guaranteed to be unique (such as Social Security Number in a table along with no more than one record per person) or it could be provided through the DBMS (such as a globally unique identifier, or GUID, in Microsoft SQL Server).

Candidate Key: A candidate key is a combination of attributes in which can be uniquely used to recognize a database record without any extraneous data. Each table may have one or more candidate keys. One of these candidate keys is chosen as the table primary key.

Alternate Key: An alternate key (or secondary key) is any candidate key that is not selected to be the primary key (PK).

For instance, a relational database along with a table "employee" could have attributes like

"employee_id", "bank_acct_no", and so on. In this case, both "employee_id" and
"bank_acct_no" serve as unique identifiers for a given employee, and could therefore arguably be used for a primary key. Therefore, both of them are known as "candidate keys". If, for example, "bank_acct_no" was chosen as the primary key, "employee_id" would become the alternate key

Secondary Key: Same as above

In above table SID is a primary key, SID and Regno are candidate keys and SIDor Regno are alternate or secondary key.