In SQL, what is the main difference between the UNION and UNION ALL set operators?

Introduction / Context:
Set operators in SQL, such as UNION and UNION ALL, are used to combine the results of two or more SELECT statements. Although their names are similar, they behave differently in terms of duplicate handling and performance. This question tests whether you can identify the core difference between UNION and UNION ALL.

Given Data / Assumptions:

• We have two SELECT statements that produce compatible result sets, with the same number of columns and compatible data types.
• We use either UNION or UNION ALL to merge the results.
• The options mention duplicate handling, data type restrictions, join behavior, sorting, and performance.
• We assume basic familiarity with SELECT queries.

Concept / Approach:
UNION performs a set union operation, which means it combines the results of two queries and removes duplicate rows from the final output. This typically requires extra work such as sorting or hashing to detect duplicates. UNION ALL simply concatenates the results of the two queries without removing duplicates, so all rows are included, including any duplicates. Because UNION ALL does less work, it is usually faster than UNION.

Step-by-Step Solution:
Step 1: Recall that UNION returns distinct rows from the combined result sets. Step 2: Remember that UNION ALL returns all rows from both result sets, including duplicates. Step 3: Recognize that this difference in duplicate handling often makes UNION ALL more efficient, especially for large result sets. Step 4: Compare this behavior with the options and see that option a accurately describes both duplicate handling and performance implications. Step 5: Confirm that UNION and UNION ALL do not change data types or act as join operators.

Verification / Alternative check:
Imagine two tables that each contain a row with the same value. If you run SELECT value FROM table1 UNION SELECT value FROM table2, the result contains one row with that value. If you run the same with UNION ALL, the result contains two rows. This simple experiment clearly shows the difference in duplicate handling and supports the explanation in option a.

Why Other Options Are Wrong:
UNION can only combine numeric columns, whereas UNION ALL can combine any data type is incorrect because both require compatible data types but are not limited to numeric columns. UNION performs an inner join between tables, whereas UNION ALL performs a left join is wrong because UNION and UNION ALL are set operators, not join operations. UNION sorts the result automatically, whereas UNION ALL always returns rows in random order with no duplicates misrepresents actual behavior; while some implementations may sort to eliminate duplicates, sorting is not the logical definition. There is no difference; UNION and UNION ALL are exact synonyms in all databases ignores the crucial duplicate handling distinction.

Common Pitfalls:
A frequent mistake is using UNION by default even when duplicate elimination is not required, introducing unnecessary overhead. Another issue is forgetting that UNION ALL can change the meaning of results when duplicates matter. Always consider whether duplicates are meaningful in your specific reporting or analysis needs.

Final Answer:
The main difference is that UNION removes duplicate rows from the combined result, whereas UNION ALL includes all rows including duplicates and is often faster.