Adventures in Machine Learning

Unlocking the Power of CTEs: Improved Readability and Performance in SQL

Introduction to CTEs

As data sets become increasingly complex, its essential for developers and analysts to have tools at their disposal to make their work easier. Common Table Expressions, or CTEs, is one such tool that has gained popularity in recent years.

It allows for more streamlined, efficient, and readable SQL code. In this article, we will explore what CTEs are and how they can benefit you.

What Are CTEs? Common Table Expressions are a type of named subquery in SQL.

They were first introduced in the SQL:1999 standard and have since become a widely used feature in various relational database management systems (RDMS) like MySQL, Oracle, and SQL Server. They provide a temporary result set that can be accessed within a query.

Benefits and Syntax

One of the significant benefits of using CTEs in place of regular subqueries is increased readability. Standard SQL subqueries can become complex and convoluted, making it difficult to understand the intention behind each statement.

With CTEs, you can define a subquery in a separate block of code and give it a name, making it easier to read. Additionally, using CTEs can help query performance since it executes once and is then called multiple times.

The general syntax for creating a CTE is to define the cte_name and cte_definition using the WITH clause. The cte_definition is the SELECT statement that will define the temporary result set.

How to Use CTEs

Now that weve gone over what CTEs are let’s look at how to use them in practice with an example. Consider a company with an employee database that includes employee information such as experience_years, salary, and manager_id.

We want to calculate the average salary for employees in the Accounting department, excluding the salary of their manager.

WITH Accounting AS (


FROM employees

WHERE department = ‘Accounting’


SELECT AVG(salary) AS department_average

FROM Accounting

WHERE manager_id NOT IN (

SELECT employee_id

FROM Accounting


In this example, we are defining a CTE named Accounting, which is a subquery that retrieves all employees in the Accounting department. We then use the CTE to select the average salary of the employees in the Accounting department, minus the salary of their manager.


In conclusion, CTEs are an essential tool for developers and analysts working with relational databases. They provide a way to define named subqueries, increasing the readability of SQL code, and improving query performance.

This article has only scratched the surface of what CTEs can do, so keep exploring and incorporating them into your work to see their full potential. 3) CTEs vs.


When it comes to SQL, both CTEs and subqueries have their uses, but which one is better? Lets dive into readability and organization, as well as execution speed and advantages.

Readability and Organization

One of the most significant benefits of CTEs over subqueries is that they increase the readability and organization of SQL code. With subqueries, developers and analysts can get bogged down by nested queries, making it challenging to understand the intention behind each statement.

By giving subqueries a name and creating separate code blocks using CTEs, it becomes easier to write and read complex SQL queries. CTEs also provide a layer of abstraction, which enables developers to define and reference a single named query in multiple places throughout a single query.

This layered approach helps break up complex SQL queries into smaller, more readable sections, significantly improving code structure.

Execution Speed and Advantages

One of the significant benefits of using CTEs is the improvement in query performance. Instead of running a subquery multiple times, a CTE executes once and is then called by the main query multiple times.

This makes it faster than subqueries, which can be executed multiple times in more complex queries, slowing down the entire query process. CTEs also have some significant advantages over subqueries in terms of functionality.

Developers can reuse a CTE multiple times within a query or even across multiple queries. This feature makes them an ideal tool for multi-use queries where developers want to avoid replication of code.

Additionally, they simplify complex reporting queries that require multiple subqueries or nested queries.

4) Recursive CTEs

Recursive CTEs are a powerful tool that allows engineers to solve computational problems that require traversing multiple routes or hierarchical structures. Recursive CTEs allow developers to create queries that can traverse trees and other hierarchical data structures with an undefined number of levels.

The syntax for recursive CTEs includes two parts: the anchor member and the recursive member. The anchor member is the initial query that identifies the starting point of the recursive query.

The recursive member then identifies the next step in the recursion process. The recursion continues until a specified condition is reached, allowing developers to build complex hierarchical queries.

Recursive queries can be used to solve a wide range of problems, including generating organizational charts, building decision trees, and charting the path of hierarchical structures. They can also solve complex data mining problems, including shortest path algorithms, network analysis, and predictive modeling.

Availability and Further Resources

Recursive queries can be used with PostgreSQL, MySQL, Oracle, SQL Server, and other major relational database management systems.

To learn more about recursive CTEs and other complex SQL queries, there are multiple courses available on that dive into these topics.

Recursive Queries course will teach you how to write recursive CTEs in a variety of SQL dialects, work with common hierarchical data structures, optimize your queries for performance. By mastering CTEs that are recursive, you can develop queries that are flexible, readable, and easy to maintain.


In conclusion, both CTEs and subqueries have their uses and advantages in SQL development. CTEs, in particular, offer many benefits to SQL developers through increased query readability, improved query performance, and the ability to reuse code blocks across multiple queries.

Recursive CTEs take these advantages even further, allowing developers to traverse multiple routes and build complex hierarchical data structures. As CTEs become a more popular tool in SQL development, it is essential for developers to explore the potential benefits of these advanced query techniques.

In conclusion, Common Table Expressions (CTEs) are a valuable tool in SQL development that offers many benefits to developers, including increased query readability, improved query performance, and the ability to reuse code blocks across multiple queries. Compared to subqueries, CTEs are easier to read, organize, and maintain, while recursive CTEs add the power to solve more complex hierarchical data structure problems.

By mastering CTEs, developers can build queries that are more readable, flexible, and easy to maintain, resulting in better overall database performance.

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