Question at position 35 What is the difference between a PRIMARY KEY and a UNIQUE KEY?Primary key can store null value, whereas a unique key cannot store null value.We can have only one primary key in a table while we can have multiple unique keysNone of thesePrimary key cannot be a date variable whereas unique key can be

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BlackTom AI · Accepted Answer

The question asks about the difference between a PRIMARY KEY and a UNIQUE KEY in a database table, and provides four answer options to evaluate.
Option 1: 'Primary key can store null value, whereas a unique key cannot store null value.' This is incorrect. A PRIMARY KEY cannot contain NULL values, whereas a UNIQUE constraint can allow a single NULL in some databases or may treat NULLs differently depending on the DBMS, but the general expectation is that PRIMARY KEY disallows NULLs and UNIQUE allows NULLs (with at most one NULL in some systems). The statement as written is misleading and does not accurately capture the typical behavior.
Option 2: 'We can have only one primary key in a table while we can have multiple unique keys' This is correct. A table can have exactly one PRIMARY KEY, which uniquely identifies each row and implicitly creates a NOT NULL constraint. A table can have multiple UNIQUE constraints to enforce uniqueness on other columns or combinations of columns.
Option 3: 'None of these' This is incorrect because Option 2 is a true statement, so selecting 'None of these' would be false.
Option 4: 'Primary key cannot be a date variable whereas unique key can be' This is incorrect as a general rule. The data type of a key (including date, integer, varchar, etc.) is not restricted for PRIMARY KEY versus UNIQUE in standard practice; both can be defined on various data types as long as the uniqueness requirement is satisfied (and NOT NULL for PRIMARY KEY).
In summary, the key factual takeaway is that a table has one primary key and can have multiple unique keys, which aligns with Option 2. Other options contain common misconceptions about nullability, exclusivity, or data type constraints that do not correctly describe standard relational database behavior.

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