Question

What is the difference between UNIQUE and PRIMARY KEY constraints in SQL?

a) UNIQUE allows duplicate values, while PRIMARY KEY enforces uniqueness.
b) PRIMARY KEY allows NULL values, while UNIQUE enforces uniqueness.
c) UNIQUE can be used on multiple columns, while PRIMARY KEY is only for single columns.
d) PRIMARY KEY is a combination of UNIQUE and NOT NULL constraints.

Answer 1

The PRIMARY KEY and UNIQUE constraints in SQL are used to enforce uniqueness in database tables, but they have some differences.

Step-by-step explanation:

The PRIMARY KEY and UNIQUE constraints in SQL are used to enforce uniqueness in database tables, but they have some differences.

The PRIMARY KEY constraint is used to uniquely identify each record in a table. It does not allow duplicate values, and it also enforces the NOT NULL constraint, which means that the column cannot contain NULL values. The PRIMARY KEY constraint is typically applied to a single column.

The UNIQUE constraint, on the other hand, allows duplicate values in the column, but it enforces uniqueness across all the rows in the table. It does not enforce the NOT NULL constraint, so the column can contain NULL values. The UNIQUE constraint can be applied to a single column or multiple columns in a table.