The UNIQUE constraint differs from the PRIMARY KEY constraint in what way?

Question

The UNIQUE constraint differs from the PRIMARY KEY constraint in what way?​

BlackTom AI · Accepted Answer

When evaluating how the UNIQUE constraint differs from PRIMARY KEY, we must inspect each statement carefully.
Option A: 'The UNIQUE constraint does not allow NULL values.' This is incorrect as a general rule; UNIQUE allows NULLs in many SQL dialects, meaning a column with a UNIQUE constraint can contain NULL values (often multiple NULLs). The primary key, by contrast, disallows NULLs.
Option B: 'The UNIQUE constraint can be created at either the column level or the table level.' This is true in many database systems and is a feature of UNIQUE constraints. It is a legitimate characteristic but does not capture the fundamental operational difference between UNIQUE and PRIMARY KEY with respect to NULL handling, which is the core distinction being asked.
Option C: 'The UNIQUE constraint allows NULL values.' This is the key differentiator highlighted here: unlike PRIMARY KEY, which must be NOT NULL, UNIQUE allows NULLs in most implementations. This reflects the primary contrast in how these constraints manage missing data.
Option D: 'The UNIQUE constraint ensures that a specific condition is true before a data value is added to a table.' This describes a CHECK constraint, not UNIQUE. CHECK enforces a boolean condition for insert/update, whereas UNIQUE enforces distinctness of values in a column or set of columns.
In summary, while B is a true statement about how UNIQUE can be defined, the question asks for the difference from PRIMARY KEY, and the defining difference is that UNIQUE allows NULL values (Option C). The other options either describe other constraints or misstate the nature of UNIQUE.

The UNIQUE constraint differs from the PRIMARY KEY constraint in what way?Single choice

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