SQL Server Union vs Union All

Hello Dev, in this article we will be discussing the differences between SQL Server’s Union and Union All, two of the most commonly used SQL operators. We will examine the various features of each operator and how they affect the performance and functionality of your SQL Server queries. By the end of this article, you will have a better understanding of how to use these operators effectively in your queries.

Union Operator

The Union operator is used to combine the result sets of two SQL queries into a single result set. The resulting set contains only unique rows, which means that any duplicates are removed. The Union operator works by comparing the rows in each result set and removing any duplicates based on their values. This makes Union operator ideal for combining results from two tables that have the same structure.

How does Union work?

To understand how Union works, let’s consider two tables, Table A and Table B, with the following data:

Table A
ID	Name	Age
1	John	25
2	Jane	30
3	Mark	35

Table B
ID	Name	Age
4	John	25
5	Mary	30
6	Lucas	35

If we want to combine the results of the two tables using the Union operator, we would use the following SQL query:

SELECT * FROM TableA UNION SELECT * FROM TableB

The resulting table would be:

Result Table
ID	Name	Age
1	John	25
2	Jane	30
3	Mark	35
4	John	25
5	Mary	30
6	Lucas	35

When should you use the Union operator?

The Union operator is ideal when you want to combine the results of two tables that have the same structure and you don’t want any duplicate rows. Additionally, the Union operator can be used to combine the result sets of two tables that have different structures, as long as the columns that are being combined are of the same data type.

However, the Union operator can be slower than the Union All operator because it requires SQL Server to perform a distinct operation. If you know that the result sets of the two queries will not contain any duplicates, you can use the Union All operator instead.

Union All Operator

The Union All operator is used to combine the result sets of two SQL queries into a single result set. The resulting set contains all rows from both result sets, which means that any duplicates are not removed. The Union All operator works by simply combining the two result sets, without any comparison or removal of duplicates.

How does Union All work?

To understand how Union All works, let’s consider two tables, Table A and Table B, with the following data:

Table A
ID	Name	Age
1	John	25
2	Jane	30
3	Mark	35

Table B
ID	Name	Age
4	John	25
5	Mary	30
6	Lucas	35

If we want to combine the results of the two tables using the Union All operator, we would use the following SQL query:

SELECT * FROM TableA UNION ALL SELECT * FROM TableB

The resulting table would be:

Result Table
ID	Name	Age
1	John	25
2	Jane	30
3	Mark	35
4	John	25
5	Mary	30
6	Lucas	35

When should you use the Union All operator?

The Union All operator is ideal when you want to combine the results of two tables and you don’t care about any duplicates or if you want to retain duplicates. Additionally, the Union All operator can be faster than the Union operator because it does not require SQL Server to perform a distinct operation.

FAQs

What is the difference between Union and Union All?

The main difference between Union and Union All is that the Union operator removes any duplicate rows from the result set, while the Union All operator retains all rows from both result sets, including any duplicates. Additionally, the Union operator can be slower than the Union All operator because it requires SQL Server to perform a distinct operation.

When should I use Union vs Union All?

You should use Union when you want to combine the results of two tables that have the same structure and you want to remove any duplicate rows from the result set. You should use Union All when you want to combine the results of two tables and you don’t care about any duplicates or if you want to retain duplicates.

Which operator is faster, Union or Union All?

Union All is generally faster than Union because it does not require SQL Server to perform a distinct operation. However, the performance difference will depend on the specific queries and the amount of data being queried.

Can I combine more than two result sets using Union or Union All?

Yes, you can combine more than two result sets using Union or Union All. Simply include additional Select statements separated by the keyword Union or Union All, depending on which operator you want to use.

Can I use Union and Union All in the same query?

Yes, you can use Union and Union All in the same query. Simply include both operators in separate Select statements within the same query.