Question at position 13 What is the difference between a PRIMARY KEY and a UNIQUE KEY?Primary key cannot be a date variable whereas unique key can bePrimary key can store null value, whereas a unique key cannot store null value.None of theseWe can have only one primary key in a table while we can have multiple unique keys

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BlackTom AI · Accepted Answer

Question restatement: What is the difference between a PRIMARY KEY and a UNIQUE KEY? Options include a mix of factual claims, and we must assess each for accuracy before identifying the correct statement.

Option 1: 'Primary key cannot be a date variable whereas unique key can be' This is misleading. A primary key can be of any data type that uniquely identifies a row (including dates), as long as it is NOT NULL and unique. The statement incorrectly restricts the PK to not be a date type and ignores the broader rule about data types and nullability.

Option 2: 'Primary key can store null value, whereas a unique key cannot store null value.' In most relational databases, a primary key must be NOT NULL and uniquely identify each row. A unique key, on the other hand, can allow NULL values (depending on the DBMS, multiple NULLs may be permitted). Therefore this option contradicts standard PK rules and is inaccurate.

Option 3: 'None of these' Claiming that none of the provided statements are true would imply that both previous options are incorrect and no other correct distinction exists. Given that at least one accurate difference exists, this option is not correct.

Option 4: 'We can have only one primary key in a table while we can have multiple unique keys' This aligns with the conventional SQL rule: a table can have exactly one primary key, which enforces entity integrity, while it can have multiple unique keys to enforce uniqueness on other candidate keys. This correctly captures the standard distinction between PRIMARY KEY and UNIQUE KEY.

Summary of evaluation: Options 1 and 2 misstate fundamental properties about nullability and data types for primary keys, and option 3 is not supported by the given truths. Option 4 accurately describes the core difference between primary keys and unique keys in most relational databases, highlighting the single primary key versus multiple unique keys capability.

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