A field that gives each record a unique identity is called as:

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Accepted Answer

The correct option is (3):
The field that is specifically chosen to give each record a unique identity within a table is called the **Primary key**. This is a foundational concept in Relational Database Management Systems (RDBMS). A Primary Key is a special type of column or a set of columns that uniquely identifies every row in the table, ensuring that no two records have the same Primary Key value. Crucially, a Primary Key enforces two main integrity constraints: it must contain unique values (the unique identity part), and it cannot contain NULL values, meaning it must always have a value assigned (the entity integrity rule). This mandatory non-null property is what makes the Primary Key the definitive identifier for a record, fulfilling the requirement of giving *each* record a unique identity.

The incorrect option is (1):
A **Unique key** is a constraint that ensures all values in a column or a group of columns are different from one another, much like a Primary Key. However, the fundamental difference is that most SQL database systems (like SQL Server, MySQL, and Oracle) allow a Unique Key column to contain a single NULL value, unless the column is explicitly defined as NOT NULL. The presence of a NULL value means that the field does not always provide a complete identity, as a record with a NULL value in the Unique Key cannot be uniquely identified in the same way as a non-null key. Therefore, while it ensures uniqueness, it is not the universal field for *each* record's unique identity that the Primary Key is.

The incorrect option is (2):
**First key** is not a standard or formally recognized term in the terminology of Relational Database Management Systems (RDBMS) or database theory. Keys like Primary Key, Foreign Key, Candidate Key, and Unique Key are established concepts with specific roles, but "First key" does not correspond to any defined structural or functional constraint. This option is likely a distracter and should be disregarded in a formal examination setting on database principles.

The incorrect option is (4):
A **Candidate key** is any column or set of columns in a table that can uniquely identify every tuple (row) without any redundant data. The key phrase is "can uniquely identify." A Primary Key is chosen by the database designer from the set of all Candidate Keys. Therefore, while a Candidate Key is a *potential* unique identifier, the Primary Key is the one officially *designated* to perform the role of providing the unique identity for the table. Every Primary Key is a Candidate Key, but not every Candidate Key is the Primary Key.