A unique index is a database constraint that ensures all values in the indexed column or columns are distinct. When a unique index is applied to a table, the database management system (DBMS) automatically rejects any INSERT or UPDATE operation that would create a duplicate value in the specified column(s). This directly enforces the developer's requirement to prevent repeated values, thereby maintaining data integrity.

B. Single column: This describes an index based on one column, which can be either unique or non-unique. It does not inherently prevent duplicates.

C. Implicit: This refers to an index created automatically by the DBMS, typically for a primary key or unique constraint, not the type of constraint itself.

D. Composite: This describes an index that includes multiple columns. A composite index can be unique, but its primary definition is its multi-column structure, not its uniqueness property.

1. Oracle Database Concepts

21c

Chapter 4: Schema Objects

Section: "Unique and Nonunique Indexes". The documentation states

"A unique index guarantees that no two rows of a table have duplicate values in the key column or columns."

2. PostgreSQL 16 Documentation

Chapter 11: Indexes

Section 11.7: "Unique Indexes". The documentation specifies

"Indexes can also be used to enforce uniqueness of column values... A unique index prevents duplicate values from being entered into the table."

3. Microsoft SQL Server Documentation

"Create Unique Indexes". The guide states

"A unique index ensures that the index key contains no duplicate values and therefore every row in the table is in some way unique."

4. Ramakrishnan

R.

& Gehrke

J. (2003). Database Management Systems (3rd ed.). In Chapter 8

"Storage and Indexing

" the concept of unique indexes is introduced as a mechanism to enforce uniqueness constraints efficiently. (This is a standard university textbook for database courses).