As a SQL Server developer, it is important to understand the difference between the UNION and UNION ALL operators. These operators are used to combine the results of two or more SELECT statements into a single result set. In this article, we will explore the concepts of UNION and UNION ALL and discuss when to use each one.

What is UNION?

The UNION operator is used to combine the result sets of two or more SELECT statements into a single result set. It removes duplicate rows from the final result set. In other words, if a row appears in multiple SELECT statements, it will only appear once in the final result set.

Here is an example:

/* Create First Table */
DECLARE @Table1 TABLE (Col INT)
INSERT INTO @Table1 SELECT 1
INSERT INTO @Table1 SELECT 2
INSERT INTO @Table1 SELECT 3
INSERT INTO @Table1 SELECT 4
INSERT INTO @Table1 SELECT 5

/* Create Second Table */
DECLARE @Table2 TABLE (Col INT)
INSERT INTO @Table2 SELECT 1
INSERT INTO @Table2 SELECT 2
INSERT INTO @Table2 SELECT 6
INSERT INTO @Table2 SELECT 7
INSERT INTO @Table2 SELECT 8

/* Result of Union operation */
SELECT Col 'Union' FROM @Table1 UNION SELECT Col FROM @Table2

In this example, the UNION operator combines the rows from @Table1 and @Table2, removing any duplicate rows. The result set is sorted in ascending order.

What is UNION ALL?

The UNION ALL operator is similar to the UNION operator, but it does not remove duplicate rows from the final result set. It simply combines all rows from all SELECT statements into a single result set. This means that if a row appears in multiple SELECT statements, it will appear multiple times in the final result set.

Here is an example:

/* Result of Union All operation */
SELECT Col 'UnionAll' FROM @Table1 UNION ALL SELECT Col FROM @Table2

In this example, the UNION ALL operator combines the rows from @Table1 and @Table2 without removing any duplicate rows. The result set is not sorted.

Performance Considerations

When deciding whether to use UNION or UNION ALL, it is important to consider the performance implications. UNION ALL is generally faster than UNION because it does not involve the additional step of removing duplicate rows. If you are certain that the result set does not contain any duplicate rows, using UNION ALL can significantly improve performance.

Conclusion

In summary, the UNION and UNION ALL operators in SQL Server are used to combine the results of multiple SELECT statements into a single result set. UNION removes duplicate rows, while UNION ALL does not. Understanding the differences between these operators and when to use each one is crucial for efficient and accurate data retrieval.