UNION and UNION ALL Operators In SQL

Hello there! If you're here, then you must be keen on learning about the UNION and UNION ALL operators in SQL. These operators are fundamental building blocks in SQL that allow you to combine the results of two or more SELECT statements. Even though they might seem straightforward at first glance, understanding the intricacies of these operators is crucial in order to harness their full power and ensure efficient querying. In this blog post, we'll be exploring what these operators are, their differences, use cases, as well as several examples to bring the concepts to life.

Understanding the Basics

Before we delve into the UNION and UNION ALL operators, it's important to grasp what a SELECT statement does in SQL. In a nutshell, a SELECT statement is used to select data from a database. The data returned is stored in a result table, called the result-set.

SELECT column1, column2, ...
FROM table_name;

Now, let's suppose that you have two SELECT statements and you want to combine their results. This is where the UNION and UNION ALL operators come in.

Introduction to UNION Operator

The UNION operator is used to combine the result-set of two or more SELECT statements. However, it removes duplicate rows from the result-set. Also, the SELECT statements within the UNION must have the same number of columns. The corresponding columns must have similar data types, and they must be in the same order.

Here's a basic syntax of a UNION:

SELECT column1, column2, ...
FROM table1
UNION
SELECT column1, column2, ...
FROM table2;

Let's assume we have two tables Orders2019 and Orders2020 with similar columns and we want to retrieve all orders from both years. This can be achieved by using UNION operator.

SELECT OrderID, CustomerID, OrderDate
FROM Orders2019
UNION
SELECT OrderID, CustomerID, OrderDate
FROM Orders2020;

This query will return a result-set that combines the rows from both Orders2019 and Orders2020 and removes any duplicates.

Digging Deeper: The UNION ALL Operator

The UNION ALL operator works in a similar way to the UNION operator, with one key difference: it does not remove duplicate rows. This operator is useful when you want to see all records from your SELECT statements, including duplicates.

Here's a basic syntax of a UNION ALL:

SELECT column1, column2, ...
FROM table1
UNION ALL
SELECT column1, column2, ...
FROM table2;

Let's use the same tables Orders2019 and Orders2020. If we want to see all orders including any duplicates, we can use UNION ALL.

SELECT OrderID, CustomerID, OrderDate
FROM Orders2019
UNION ALL
SELECT OrderID, CustomerID, OrderDate
FROM Orders2020;

UNION vs UNION ALL: Performance Considerations

The choice between UNION and UNION ALL is not merely about whether you want to see duplicates or not. It also has performance implications.

The UNION operator has to do additional work because it needs to remove duplicates. This involves sorting and comparing the rows from the combined result-set, which can be a resource-intensive process, especially for large datasets.

On the other hand, UNION ALL simply combines the rows from the result-sets of the SELECT statements. It doesn’t haveto sort or compare any rows, which makes it faster and less resource-intensive compared to UNION.

Therefore, if performance is a concern and you're sure that there won't be any duplicates, or if duplicates don't matter for your particular case, it's better to use UNION ALL.

Examples and Use Cases

Now that we've covered the basics, let's dive into a few examples to see UNION and UNION ALL in action.

Example 1: Combining Similar Tables

Imagine you have two tables: NewYork_Customers and Boston_Customers. Both tables have the same columns: CustomerID, CustomerName, and ContactNumber. You want to create a list of all customers, irrespective of the city. Here is how you can do it:

SELECT CustomerID, CustomerName, ContactNumber 
FROM NewYork_Customers
UNION
SELECT CustomerID, CustomerName, ContactNumber 
FROM Boston_Customers;

And if you want to include duplicates:

SELECT CustomerID, CustomerName, ContactNumber 
FROM NewYork_Customers
UNION ALL
SELECT CustomerID, CustomerName, ContactNumber 
FROM Boston_Customers;

Example 2: Unifying Different Types of Data

Suppose you have a Books table and a Magazines table. Both tables have an ISBN column and a Title column, but the Books table has a Author column, and the Magazines table has a Publisher column. You want to create a unified list of ISBNs and Titles. Here's how you can use UNION:

SELECT ISBN, Title FROM Books
UNION
SELECT ISBN, Title FROM Magazines;

And if you want to include duplicates:

SELECT ISBN, Title FROM Books
UNION ALL
SELECT ISBN, Title FROM Magazines;

Please note, however, that UNION or UNION ALL can only be used when the SELECT statements fetch the same number of columns and the corresponding columns are of compatible data types.

FAQ

Q: Can I use UNION with tables that have a different number of columns?

No, the UNION and UNION ALL operators require that each SELECT statement has the same number of columns.

Q: How can I sort the result-set of a UNION?

You can add an ORDER BY clause at the end of the last SELECT statement:

SELECT column1 FROM table1
UNION
SELECT column2 FROM table2
ORDER BY column1;

Q: What is the difference between JOIN and UNION?

JOIN is used to combine rows from two or more tables based on a related column. On the other hand, UNION is used to combine the result-set of two or more SELECT statements.

Q: Does UNION automatically order the result-set?

No, UNION does not automatically order the result-set. The order of the result-set from a UNION is determined by the database system's default. If you want to guarantee a particular order, use the ORDER BY clause.

Q: How can I include the duplicates in my result-set?

If you want to include duplicates in your result-set, use UNION ALL instead of UNION.

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SQL's UNION and UNION ALL operators are powerful tools that can help you manipulate and retrieve data across multiple tables. By understanding how these operators work and when to use each one, you canwrite more efficient queries and fully utilize the potential of your database.

Remember, the key difference between UNION and UNION ALL is that UNION removes duplicate rows from the result-set while UNION ALL includes duplicates. Moreover, UNION ALL is generally faster because it does not have to sort and compare rows to remove duplicates, unlike UNION.

In this post, we went through the basics of these two operators, demonstrated how they are used in various scenarios, and addressed common questions. As always, the best way to truly understand and get comfortable with these concepts is through practice. Try running your own queries using UNION and UNION ALL and see the results for yourself.

We hope that this post has been helpful for you to understand the UNION and UNION ALL operators in SQL. As you continue your journey in SQL, always remember that the key to mastering any programming language or tool is through continuous learning and practice.

Happy querying!