In DB2 SQL, what is the difference between UNION and UNION ALL when combining the results of two SELECT queries?

Introduction / Context:
DB2, like other relational databases, supports set operations that allow you to combine the results of multiple SELECT statements. UNION and UNION ALL are two such operations. They look similar, but they differ in how they handle duplicate rows and in their performance characteristics. Understanding this difference is essential for writing correct and efficient queries in DB2.

Given Data / Assumptions:
You have two or more SELECT queries with the same number of columns and compatible data types in the corresponding positions.You want to combine the results into a single result set.You may or may not want duplicate rows to appear in the final result.DB2 supports both UNION and UNION ALL set operators.

Concept / Approach:
The UNION operator performs a distinct union of two result sets. It concatenates the rows from both queries and then removes duplicates from the combined set, similar to applying DISTINCT to the merged results. UNION ALL simply concatenates the result sets from the participating SELECT statements without removing duplicates. Because UNION ALL skips the de duplication step, it is usually faster and uses fewer resources, especially for large data sets. The choice between them depends on whether duplicates are meaningful for your analysis.

Step-by-Step Solution:
First, recall that UNION and UNION ALL both require the same number of columns in each SELECT and compatible data types and column order.Next, remember that UNION performs an implicit distinct operation, which means that if the same row appears in both input sets, it will appear only once in the output.Then, recognize that UNION ALL does not remove duplicates. If a row appears twice, it will appear twice in the final result, and row counts will simply add up.After that, review the answer choices and identify option A as the one that describes this relationship correctly.Finally, confirm that the other options introduce incorrect behavior, such as updates or deletes, that are not associated with UNION operations.

Verification / Alternative check:
DB2 and general SQL references show syntax examples like SELECT column_list FROM table1 UNION SELECT column_list FROM table2, and they note that UNION removes duplicates. They also show SELECT column_list FROM table1 UNION ALL SELECT column_list FROM table2, with explicit comments stating that duplicates are retained. Performance tuning guides commonly recommend using UNION ALL when duplicate removal is not needed because it avoids sorting or hashing operations. This documentation aligns with option A.

Why Other Options Are Wrong:
Option B confuses UNION with data modification operations, which are done with INSERT, UPDATE, or DELETE, not with set operations. Option C claims a difference based on database instances, which is not how UNION works; cross database queries depend on connectivity and naming, not on UNION versus UNION ALL. Option D restricts UNION and UNION ALL by data type, which is incorrect; both work with any compatible column types. Option E claims that UNION ignores column order, which is false; both operators require the same column order in participating SELECT statements.

Common Pitfalls:
A common mistake is using UNION when UNION ALL would be sufficient, causing unnecessary de duplication and slower queries. Another pitfall is using UNION ALL when duplicates are not desired, which can inflate counts and mislead analysis. Developers sometimes also forget that column names in the final result come from the first SELECT in the UNION. Being deliberate about choosing UNION or UNION ALL based on whether duplicates should be removed improves both correctness and performance of DB2 queries.

Final Answer:
The correct answer is: UNION removes duplicate rows in the combined result set, whereas UNION ALL returns all rows including duplicates without performing a distinct operation.