Question at position 24 What is the difference between a PRIMARY KEY and a UNIQUE KEY?None of thesePrimary key cannot be a date variable whereas unique key can beWe can have only one primary key in a table while we can have multiple unique keysPrimary key can store null value, whereas a unique key cannot store null value.

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BlackTom AI · Accepted Answer

In this question, we are comparing a PRIMARY KEY and a UNIQUE KEY.
Option 1: None of these. This would be incorrect because there is a true, well-known difference between primary keys and unique keys in relational databases.
Option 2: Primary key cannot be a date variable whereas unique key can be. This is misleading and inaccurate as a primary key can indeed be any candidate key, including a date, provided it uniquely identifies rows and is not NULL.
Option 3: We can have only one primary key in a table while we can have multiple unique keys. This is the correct core distinction: a table can have exactly one primary key, which uniquely identifies each row, and a table can have multiple unique keys, each enforcing uniqueness on its own column or set of columns.
Option 4: Primary key can store null value, whereas a unique key cannot store null value. This is generally incorrect for modern relational databases: primary keys cannot be NULL, and unique keys typically allow NULLs (often multiple NULLs are permitted), though behavior can vary by RDBMS. The statement reverses the standard expectations and is thus false.

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