MCQ IN COMPUTER SCIENCE & ENGINEERING

Detailed explanation-1: -A primary key is a column of table which uniquely identifies each tuple (row) in that table. Primary key enforces integrity constraints to the table. Only one primary key is allowed to use in a table. The primary key does not accept the any duplicate and NULL values.

Detailed explanation-2: -The UNIQUE constraint allows NULL values, but the PRIMARY KEY constraint does not allow null values.

Detailed explanation-3: -The main difference between a primary key vs unique key is that a primary key is a key that uniquely identifies each record in a table but cannot store NULL values. In contrast, a unique key prevents duplicate values in a column and can store NULL values.

Detailed explanation-4: -Details. You can insert NULL values into columns with the UNIQUE constraint because NULL is the absence of a value, so it is never equal to other NULL values and not considered a duplicate value. This means that it’s possible to insert rows that appear to be duplicates if one of the values is NULL .