........... constraint ensures that no two 'rows' have same value in specified column/columns.

Question

Accepted Answer

The correct option is (2):
The **UNIQUE** constraint is a fundamental concept in Relational Database Management Systems (RDBMS) designed specifically to enforce entity integrity by guaranteeing that all values in a column, or a group of columns, are distinct. Its primary function is to ensure that no two rows within the table will have identical values for the constrained column(s), thereby fulfilling the requirement stated in the question. Unlike the **PRIMARY KEY** constraint, which is essentially a combination of **UNIQUE** and **NOT NULL**, the **UNIQUE** constraint allows for multiple *NULL* values in the column, as *NULL* is generally considered a lack of value and therefore not equal to any other *NULL* value. This mechanism is crucial for maintaining data quality and allowing the constrained column(s) to serve as an alternate key for uniquely identifying records in the table.

The incorrect option is (1):
The **Default** constraint is entirely unrelated to preventing duplicate data across rows. Its purpose is to provide a default value to a column when an *INSERT* statement does not explicitly supply a value for that column. For example, setting the default value of a 'status' column to 'Pending' ensures that every new record has an initial status without requiring user input. It only addresses the presence of a value, not the distinctiveness of that value compared to others in the table. The constraint focuses on data completion, not on the uniqueness of the data, and therefore does not prevent two rows from having the same value.

The incorrect option is (3):
The **Check** constraint is used to limit the range of values that can be accepted by a column, based on a logical condition that must evaluate to *TRUE*. For instance, you could use a **CHECK** constraint to ensure that an 'Age' column only accepts values greater than 18. This constraint is excellent for enforcing business rules and validating the integrity of a single data point within a record. However, it only checks the value against a static condition defined during table creation. It does not compare the value in the current row against values in other existing rows, which is the mechanism necessary to prevent duplicates.

The incorrect option is (4):
The statement "All options are wrong" is incorrect because Option (2), the **UNIQUE** constraint, precisely matches the functional requirement described in the question. The UNIQUE constraint is the standard, well-defined SQL mechanism whose sole purpose is to ensure that "no two 'rows' have same value in specified column/columns." Since one of the provided options accurately describes the required function, the blanket statement that all options are wrong is factually incorrect in the context of database integrity constraints.