Union SQL Server: A Comprehensive Guide for Dev

Greetings Dev! Are you looking to learn more about union in SQL Server? You’ve come to the right place! In this article, we’ll be discussing everything you need to know about union in SQL Server. From the basics to advanced techniques, we’ve got you covered.

What is Union in SQL Server?

Before we get into the details, let’s first understand what union is in SQL Server. Union is a set operator that combines the result sets of two or more SELECT statements into a single result set. This is particularly useful when you want to combine data from multiple tables or views.

How Does Union Work in SQL Server?

Union works by removing duplicates from the result set. When you use the union operator, SQL Server compares each row in the first result set to each row in the second result set. If a row is duplicated, SQL Server eliminates it from the combined result set.

It’s important to note that when using union, the number and data types of the columns in each SELECT statement must be the same. Otherwise, SQL Server will throw an error.

When to Use Union in SQL Server?

Union is particularly useful when you want to combine data from multiple tables or views. For example, let’s say you have two tables – customers and orders – and you want to see a list of all customers and their associated orders. You could use the union operator to combine the results of two SELECT statements:

customers
John Smith
Jane Doe

orders
John Smith – Order 1
John Smith – Order 2
Jane Doe – Order 1

The resulting query would look something like this:

SELECT Name FROM customersUNIONSELECT CustomerName + ' - ' + OrderName FROM orders

This would combine the two result sets and remove any duplicates, giving you the following result:

Result Set
John Smith
Jane Doe
John Smith – Order 1
John Smith – Order 2
Jane Doe – Order 1

Union vs. Union All in SQL Server

Now that you understand the basics of union in SQL Server, let’s take a look at the difference between union and union all. Union all is another set operator that combines the result sets of two or more SELECT statements into a single result set. The key difference between union and union all is that union all does not remove duplicates.

While union is great for removing duplicates, it can also be more resource-intensive than union all. This is because union has to perform a comparison of each row in each result set, while union all simply combines the result sets without any additional processing.

When to Use Union All in SQL Server?

If you don’t care about duplicates and want to combine result sets quickly and efficiently, then union all is the way to go. However, if you need to remove duplicates and don’t mind the additional processing time, then union is the better choice.

Using Union in SQL Server with Examples

Now that you understand the basics of union and union all in SQL Server, let’s take a look at some real-world examples. In this section, we’ll walk through some common scenarios where you might use the union operator.

Combining Data from Multiple Tables

As we mentioned earlier, union is particularly useful when you want to combine data from multiple tables. Let’s take a look at an example:

employees
John Smith
Jane Doe

contractors
Bob Johnson
Jill Thompson

The following query will combine the employees and contractors tables:

SELECT Name FROM employeesUNIONSELECT Name FROM contractors

The resulting query will look like this:

Result Set
John Smith
Jane Doe
Bob Johnson
Jill Thompson

Creating a List of Unique Values

Another common use case for union in SQL Server is creating a list of unique values. Let’s say you have a table that contains duplicate values, and you want to create a list of unique values. You could use the union operator to remove duplicates:

inventory
Apples
Oranges
Apples
Bananas

The following query will remove duplicates and create a list of unique values:

SELECT Fruit FROM inventoryUNIONSELECT Vegetables FROM inventory

The resulting query will look like this:

Result Set
Apples
Oranges
Bananas
Broccoli
Carrots

FAQs

What is the difference between union and union all in SQL Server?

The key difference between union and union all is that union removes duplicates from the result set, while union all does not.

Can you combine more than two SELECT statements with union in SQL Server?

Yes, you can combine as many SELECT statements as you like with union in SQL Server.

What happens if the number and data types of the columns in each SELECT statement are different?

If the number and data types of the columns in each SELECT statement are different, SQL Server will throw an error.

Is union more resource-intensive than union all in SQL Server?

Yes, union can be more resource-intensive than union all because it has to perform a comparison of each row in each result set to remove duplicates.

When should I use union in SQL Server?

Union is particularly useful when you want to combine data from multiple tables or create a list of unique values. You should use union when you need to remove duplicates and don’t mind the additional processing time.

Conclusion

By now, you should have a solid understanding of union in SQL Server. Whether you’re combining data from multiple tables or creating a list of unique values, union can be a powerful tool in your SQL Server arsenal. So go forth and explore the world of union in SQL Server!