Question at position 26 What is the difference between a PRIMARY KEY and a UNIQUE KEY?Primary key cannot be a date variable whereas unique key can beNone of thesePrimary 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 keys

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BlackTom AI · Accepted Answer

First, we restate the question and enumerate the answer options to set the scene for analysis.
Question: What is the difference between a PRIMARY KEY and a UNIQUE KEY?
Options:
1) Primary key cannot be a date variable whereas unique key can be
2) None of these
3) Primary key can store null value, whereas a unique key cannot store null value.
4) We can have only one primary key in a table while we can have multiple unique keys

Now evaluating each option in turn:
Option 1: Primary key cannot be a date variable whereas unique key can be
This claim mixes data type with key type. In relational databases, a primary key can be of many data types, including dates, as long as the value uniquely identifies a row and is not NULL (depending on the system). Therefore, restricting a primary key to not be a date variable is false on general principles, and this statement is not accurate.
Option 2: None of these
This would be chosen if all the other statements were incorrect. We will assess the other options to determine if this is the case.
Option 3: Primary key can store null value, whereas a unique key cannot store null value.
This contradicts standard behavior in most DBMSs. A primary key is not allowed to be NULL because it must uniquely identify a row; many systems enforce NOT NULL on primary keys. A unique key, by contrast, may allow a single NULL value (or multiple NULLs) depending on the DBMS, but often allows NULLs. Since the statement claims the opposite of typical behavior, it is incorrect.
Option 4: We can have only one primary key in a table while we can have multiple unique keys
This statement aligns with common relational database rules: a table can have only one primary key, which can consist of one or more columns (composite key), while a table can define multiple distinct UNIQUE constraints across different columns or combinations. This captures the core difference between PRIMARY KEY and UNIQUE KEY accurately, reflecting standard database design principles.

To underscore the distinctions and common misconceptions:
- The primary key’s main role is to uniquely identify each row and to be referenced by foreign keys; this is why it cannot be NULL and why there can only be one primary key per table.
- A unique key enforces uniqueness for a set of columns, but does not necessarily imply the row cannot be referenced by foreign keys, and a table can have multiple unique constraints across different columns or column sets.
- Data type compatibility (including dates) is generally flexible for both keys, as long as the uniqueness and not-null requirements are satisfied for the primary key; thus, Option 1’s limitation about dates is misleading.

In summary, Option 4 correctly states the structural difference most closely aligned with standard database behavior, while the other options contain inaccuracies about nullability, data type restrictions, or the count of keys per table.

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