How to Learn SQL Common Table Expressions (CTEs)

Common table expressions are very useful for organizing, traversing, and improving the readability of long SQL queries. 

Moreover, recursive CTEs, which can reference themselves, solve problems that cannot be addressed with other queries. But how do you master common table expressions in SQL? In this article, I discuss the most effective strategies for learning CTEs and suggest how to address the common challenges of learning common table expressions on your own.

What Is a CTE?

After you have mastered SQL basics and can create, query, and join tables, you probably want to make your queries look even more professional. At that point, you’re ready to start learning about SQL’s modern concepts, introduced in the 2000s.

A common table expression (CTE) is one of these concepts. CTEs are applied for the better readability and organization of SQL queries. So, what is a CTE?

Basically, a CTE is a named temporary result set that can be used in subsequent SELECT, INSERT, UPDATE, or DELETE statements. You simply assign a name to a specific result set created with a usual SQL statement; then, in your subsequent SQL statement, you can refer to this result set as you would to an ordinary table.

Want a more detailed explanation? Read our beginner-friendly introduction to CTEs.

CTEs can be very useful in real-world applications, since they:

• Make long queries more readable and easier to follow.
• Improve the organization of complex queries and reports.
• Assist with processing hierarchical data (in the case of recursive queries).

Learn more about when to use common table expressions in our comprehensive guide.

You are likely excited about this relatively new technique. But what is the most effective way to master CTEs?

It’s time to review the best strategies for becoming a CTE pro.

Effective Strategies for Learning SQL Common Table Expressions

Common table expressions are usually quite straightforward to use. However, recursive CTEs (a.k.a. recursive queries) are often challenging to grasp and properly apply. Thus, it’s important to have a deep understanding of CTEs. This can be achieved only with a structured approach to learning this new technique.

Let’s now discuss how you can get this solid knowledge of common table expressions.

1. Read beginner-friendly articles on Common Table Expressions. These articles will introduce you to CTE syntax, common CTE applications, and use case examples for real-world business scenarios. Getting this foundation is a critical step before moving to the next learning stages.

2. Dive into advanced articles on recursive queries. After you’ve got the basic understanding of common table expressions, you’re ready to move on to recursive CTEs and queries. These queries refer back to themselves until a certain threshold is reached. As you only need to write the code once, these queries are very convenient when you need to perform the same function numerous times.

   Given their functionality, recursive CTEs are very effective when working with hierarchical data, like organizational or folder/subfolder structures. However, it can be quite challenging to understand how to apply these queries properly.

   Before jumping into practice, I recommend you read through the recursive CTE syntax and examine several well-explained examples of their application. This will give you a clear understanding of how recursive queries work under the hood.

3. Practice CTEs using interactive exercises. Now that you have a good understanding of common table expressions, it’s time for some coding. Interactive exercises are usually the best starting point for practicing any new SQL topic.

   LearnSQL.com offers a comprehensive course on CTEs; it’s called Recursive Queries and it contains 114 interactive exercises. Despite the name, the course covers not only recursive queries but all kinds of common table expressions. And it starts with the simplest ones so you can build your skills gradually. In this course, you’ll learn how to use basic common table expressions, how to organize long queries with simple and nested CTEs, how to deploy recursive CTEs, and how to move through hierarchical data using recursive queries.

4. Explore examples of SQL reports. Browsing through SQL reports prepared by your colleagues can be another good way to learn CTEs. If you don’t have access to real SQL reports, search for examples on the Internet. It’s always a good idea to see how common table expressions are applied in the real world.
5. Read your database’s documentation. As with any SQL concept, reading the documentation of your database is highly recommended. While simple CTEs have the same syntax regardless of the database, recursive CTEs’ syntax varies across databases. For example, in PostgreSQL and MySQL, you’ll need to use WITH RECURSIVE instead of WITH when writing recursive queries. So be sure to read the documentation for the database you use!
6. Practice CTEs in realistic scenarios. As you know, the shortest path to becoming an SQL pro is through lots of coding. Reading about CTEs is not enough – you need to practice writing CTEs in realistic scenarios. Maybe you can start applying simple CTEs in the SQL queries you write at work. If that’s not an option, think about doing some side projects using SQL CTEs.

Common Challenges When Learning CTEs on Your Own

Even though common table expressions aren’t super complex, you can encounter challenges when learning CTEs on your own. For example, it can be hard to:

• Structure your learning material. It can be difficult to arrange your learning process so that all the important nuances of SQL CTEs are covered. And organizing your material in a logical way can be hard when you don’t fully understand the topic. You may need some assistance from CTE experts. Alternatively, you could just follow the content of LearnSQL’s Recursive Queries course, where the material is structured in a thought-out way and covers all important aspects of CTEs.
• Find practice datasets. It can be difficult to find suitable datasets to practice CTEs, especially recursive CTEs. If possible, you can try practicing with the data that you use at work. Alternatively, you can leverage the datasets used in our interactive course. These datasets are realistic and resemble real-world problems.
• Come up with good exercises. Another challenge when learning CTEs on your own is coming up with good practice exercises. This is particularly true for recursive CTEs; the topic is not as simple as JOINs or GROUP BY, and it might be very difficult to come up with realistic problems to solve. Here again, I would recommend you practice with our Recursive Queries course, which is the only interactive recursive CTE course available online (at the time of writing). Bonus: You don’t need to install additional software. You practice in your browser!

Time to Learn SQL CTEs!

Here is your step-by-step guide to learning SQL CTEs:

1. Read about simple CTEs to learn the basics:
   • What Is a CTE?
   • When Should I Use a Common Table Expression (CTE)?
   • SQL CTEs Explained with Examples
   • How CTEs Work
   • How to Organize SQL Queries with CTEs
2. Read about recursive CTEs:
   • How Recursive Common Table Expressions Work
   • Long SQL Query vs. Recursive SQL Query
3. Complete an interactive course covering all the important nuances of CTEs.
4. Explore real-world SQL reports to see how SQL experts leverage the power of CTEs.
5. Check out the corresponding documentation of your database.
6. Practice CTEs with real-world job assignments.

Thanks for reading, and happy learning!