What does the UNION operation automatically eliminate in SQL?Null values.Duplicate tuples.Relations with different schemas.Operations on virtual relations.Next

1. Break Down the Problem

To determine what the UNION operation in SQL automatically eliminates, we need to consider the behavior of the UNION operation and its characteristics with respect to set theory.

2. Relevant Concepts

• UNION Operation: In SQL, the UNION operator combines results from two or more SELECT statements.
• Set Properties: In set theory, a set cannot have duplicate elements. Therefore, when performing a UNION, any duplicate rows between the result sets are removed.

3. Analysis and Detail

• Null Values: The presence of NULL values is retained in the result set if they are part of both input tables; they are treated as distinct unless they are duplicates.
• Duplicate Tuples: The primary function of the UNION operator is to eliminate duplicate tuples. If two or more rows are identical across the result sets being combined, only one instance of that row will appear in the final result.
• Relations with Different Schemas: For a UNION to be valid, the SELECT statements must return the same number of columns with similar data types, which means that the schemas of the two relations must be compatible, but they need not be identical.
• Operations on Virtual Relations: This is not directly relevant to what UNION eliminates. Virtual tables can still participate in Joins, but the operation does not inherently change this.

4. Verify and Summarize

Considering the key behavior of the UNION operation—and how it aligns with set properties—the automatic elimination that happens is notably of duplicate tuples while retaining individual NULLs unless they are identical. Hence, the correct answer to the question is:

Final Answer

Duplicate tuples.