When working with SQL Server, you may come across situations where you need to combine data from multiple tables. Two common ways to achieve this are by using the UNION and UNION ALL commands. In this article, we will explore the differences between these two commands and when to use each one.

UNION

The UNION command is used to select related information from two tables, similar to the JOIN command. However, there are a few key differences. First, when using UNION, all selected columns need to be of the same data type. Second, the UNION command only selects distinct values, meaning it eliminates duplicate rows from the result set.

UNION ALL

The UNION ALL command, on the other hand, selects all values from both tables without eliminating duplicates. This means that if there are duplicate rows in the tables, they will appear multiple times in the result set. The UNION ALL command is generally faster than UNION because it does not require the additional step of removing duplicates.

Let’s take a look at an example to better understand the difference:

/* Declare First Table */
DECLARE @Table1 TABLE (ColDetail VARCHAR(10))
INSERT INTO @Table1 SELECT 'First' UNION ALL SELECT 'Second' UNION ALL SELECT 'Third' UNION ALL SELECT 'Fourth' UNION ALL SELECT 'Fifth'

/* Declare Second Table */
DECLARE @Table2 TABLE (ColDetail VARCHAR(10))
INSERT INTO @Table2 SELECT 'First' UNION ALL SELECT 'Third' UNION ALL SELECT 'Fifth'

/* Check the data using SELECT */
SELECT * FROM @Table1
SELECT * FROM @Table2

/* UNION ALL */
SELECT * FROM @Table1 UNION ALL SELECT * FROM @Table2

/* UNION */
SELECT * FROM @Table1 UNION SELECT * FROM @Table2

In this example, we have two tables: @Table1 and @Table2. When we run the UNION ALL command, we can see that it returns all rows from both tables, including duplicates. However, when we run the UNION command, only distinct rows from both tables are retrieved.

It’s important to note that if you know that all the records returned are unique from your union, it is recommended to use UNION ALL instead of UNION. This is because UNION ALL provides faster results by skipping the step of removing duplicates.

When comparing the execution plans of UNION ALL and UNION, it is clear that UNION ALL is less expensive as it does not require a DISTINCT SORT operation.

In conclusion, the choice between UNION and UNION ALL depends on your specific requirements. If you need to combine data from multiple tables and eliminate duplicates, use UNION. If duplicates are not a concern and you want faster results, use UNION ALL.

I hope this article has helped you understand the difference between UNION and UNION ALL in SQL Server. If you have any suggestions for improvement or any questions, please let me know. I am always open to feedback and will update the article accordingly.