List of top Database Management Systems Questions on Integrity constraints

Which two constraints when applied together will produce a primary key constraint?

Which two constraints when applied together will produce a primary key constraint ?
(A) NOT NULL
(B) UNIQUE
(C) DEFAULT
(D) ZERO
Choose the correct answer from the options given below:

Which statement is used to add a constraint in existing attribute of a relation?

Consider the following statements S1 and S2 about the relational data model:
S1: A relation scheme can have at most one foreign key.
S2: A foreign key in a relation scheme $R$ cannot be used to refer to tuples of $R$.
Which one of the following choices is correct?