What do you understand by the terms candidate key and alternate key in a relational database?

What is SQL? What are different categories of commands available in SQL?

Differentiate between DDL and DML commands.

Give the terms for each of the following:

(a) Collection of logically related records.

(b) The fundamental data storage unit in a relational database.

(c) Attribute that can uniquely identify the tuples in a relation.

(d) Special value that is stored when actual data value is unknown for an attribute.

(e) An attribute which can uniquely identify tuples of the table but is not defined as primary key of the table.

(f) Software that is used to create, manipulate and maintain a relational database.