What is a Unique Constraint in SQL Server?

A UNIQUE constraint is used to avoid duplicate values but accepted null values in a column.

The UNIQUE constraint will accept $1$ NULL value only.

A table can contain multiple number of UNIQUE constraints

All of the Above

It enforces the uniqueness of the values in a set of columns, so no duplicate values are entered. The unique key constraints are used to enforce entity integrity as the primary key constraints.