Understanding Union All SQL Server for Devs

Hello Devs, in this journal article, we will learn about one of the most essential SQL Server commands, Union All. As a developer, you may already have encountered situations where you need to merge data from multiple tables into a single result set. This is where Union All comes in handy. We will discuss its usage, benefits, and limitations in detail. So, let’s get started!

What is Union All?

Union All is a command in SQL Server that combines the results of two or more SELECT statements. It is used to combine rows from different tables into a single result set. Each SELECT statement must have the same number of columns, data types, and order.

Example:

Suppose we have two tables, Employee and Customer, with the following data:

Employee
EmpID	FirstName	LastName
1	John	Doe
2	Jane	Smith

Customer
CustID	FirstName	LastName
1	Mark	Johnson
2	Sara	Lee

We can use the following SQL statement to combine the data from both tables:

SELECT EmpID, FirstName, LastName FROM Employee
UNION ALL
SELECT CustID, FirstName, LastName FROM Customer

This will result in the following output:

EmpID	FirstName	LastName
1	John	Doe
2	Jane	Smith
1	Mark	Johnson
2	Sara	Lee

Benefits of Union All

Union All has several benefits:

Combine Data from Multiple Tables

The primary benefit of Union All is its ability to combine data from multiple tables. This is useful when you need to merge data from different sources into a single result set. It can save time and effort by eliminating the need to manually merge data.

No Duplicate Rows

Union All only returns unique rows, eliminating duplicates. This is helpful when dealing with large datasets where duplicate data can be costly in terms of storage and processing power.

Fast Performance

Union All is optimized for performance and can be executed quickly, even when working with large datasets.

Limitations of Union All

While Union All is a useful command, it also has some limitations:

Column Names Must Match

The column names in each SELECT statement must match, or Union All will return an error. This can be problematic when dealing with large datasets where column names may not match across tables.

Data Types Must Match

The data types of the columns in each SELECT statement must match, or Union All will return an error. This can be problematic when dealing with datasets where data types may not match across tables.

Performance Can Be Slower with Large Datasets

While Union All is optimized for performance, it can still be slower when working with large datasets. This is especially true when performing complex data manipulations.

FAQs

What is the difference between Union and Union All in SQL Server?

The primary difference between Union and Union All is that Union only returns unique rows, while Union All returns all rows, including duplicates.

When should I use Union All?

You should use Union All when you need to merge data from multiple tables into a single result set and do not need to eliminate duplicates.

Can I use Union All with more than two tables?

Yes, you can use Union All with more than two tables. You can combine the results of as many SELECT statements as needed.

Can I use Union All with different data types?

No, the data types of the columns in each SELECT statement must match for Union All to work.

Can I use Union All to combine columns with different names?

No, the column names in each SELECT statement must match for Union All to work.

Conclusion

Union All is a powerful tool in SQL Server that enables developers to combine data from multiple tables into a single result set. It is optimized for performance and can be executed quickly, even when working with large datasets. While it has some limitations, such as requiring matching column names and data types, its benefits make it an essential command for developers. We hope this article has helped you understand the basics of Union All and how it can be used in your projects.