Which of the following is a component of a database system?

Database

DBMS

Front-end applications

All of the above

Please select the correct statement about the primary key in a relational database:

At some time, two rows in the same table can have the same value for the primary key.

A primary key must be a simple attribute.

A primary key attribute (column) is not allowed to accept a null value.

A primary key is not necessarily a candidate key.