Adventures in Machine Learning

Simplifying Complex SQL Queries with Common Table Expressions

Introduction to SQL WITH Clauses

Structured Query Language (SQL) is an essential tool for data management, retrieval, and analysis. When working with SQL, developers are often faced with complex queries that are difficult to read and understand.

This is where Common Table Expressions (CTEs), also known as SQL WITH clauses, come in handy. CTEs have become increasingly popular in recent years due to their readability, structure, and flexibility.

In this article, we will explore the benefits and syntax of SQL WITH clauses, as well as their importance in modern data management.

Definition and benefits of WITH clauses

A WITH clause is a named subquery that is defined within the main query and can be referenced in several parts of the query. It is often used to define a temporary result set that can be referred to later in the same query.

The WITH clause can be thought of as a temporary table that exists only for the duration of a query. The use of CTEs offers several benefits, including readability, maintainability, and self-documentation.

When a query requires multiple CTEs, using them can create a more structured and organized process. By using SQL WITH clauses, the various computations in a query can be separated and organized into smaller, more manageable pieces.

This makes it easier to read and understand complex queries, especially when dealing with multiple nested subqueries.

Syntax and types of CTEs

Named subqueries within a WITH clause are defined in a way that is similar to creating a new table. The syntax involves the use of the WITH keyword, followed by the name of the CTE, and then a SELECT statement that defines the data set.

The SELECT statement can contain any number of merged tables, subqueries, or other complex queries. There are several different types of CTEs, including multiple CTEs, nested CTEs, recursive CTEs, and common table expressions with a select statement.

Multiple CTEs are used when a query requires more than one named subquery, while nested CTEs refer to the use of a CTE within another CTE. Recursive CTEs, on the other hand, are used to perform self-joins within the same CTE.

Importance of Using WITH Clauses

Improved Readability

In modern data management, organizations deal with huge amounts of data. Large and unwieldy data sets can make it difficult to know when a particular computation ends, resulting in poor readability.

The use of CTEs improves the legibility of a query by breaking it into more manageable and logical units. It helps developers create a more readable query by defining parts of the computation separately and then joining them to the final result.

Well-written SQL WITH clauses offer self-documenting code. Since the CTE is named and its purpose is explicitly defined, it is easier for other developers to understand the query’s purpose.

This documentation also helps with code maintenance over time. When database changes occur, developers can easily understand which parts of the query need to be updated.

Breaking Computations into Parts

SQL WITH clauses make it easier to break a computation into parts and create interim results. This allows developers the flexibility to choose the logical order of evaluation that best fits their query.

Given the flexibility, CTEs can create temporary tables or virtual tables for intermediate computations. This can make queries more efficient, as computations do not need to be repeated.

In other situations, using a table can be expensive. Say the aggregated data is too large or there are no permissions to write to a table.

In such cases, running interim computations that will be used later can still be done with SQL WITH clauses.

Nesting Computations

Data systems usually have more than one relation to query. A query can depend on data from various sources, which may need to be joined, filtered, or merged.

This decoupling of query sources can lead to layered SQL WITH clauses. Each layer can act as a reference or reference point to the parent layer.

This means that following a problem through layers in a query will be more efficient when using SQL WITH clauses. Layers are less coupled since the same query is split into different components to enhance readability and clarity.

SQL WITH clauses also enable a join to itself, which is referred to as recursive SQL WITH clauses. Recursive CTEs start with a base case of the query, a term that refers to a recursive common subexpression’s beginning.

The base case is the problem that needs recursion. Iterating over and over again is then done until a condition is met, and the terminal case is returned.

Recursion is common in mathematical and graph-based problems and is an essential factor in producing more than one result.


When working with large data sets, SQL can be a powerful tool for quick and efficient data management. However, the sheer size of data can make writing queries extremely complex and difficult to understand.

SQL WITH clauses, also known as Common Table Expressions, can simplify complex queries and make them more readable, maintainable, and organized. They offer various benefits to data management, including improved readability and maintainability, breaking computations into parts, and nesting computations.

Therefore, as developers continue to rely on SQL for data management and analysis, SQL WITH clauses have become an essential tool for better query construction.

Practical Applications of SQL WITH Clauses

SQL WITH Clauses, also known as Common Table Expressions (CTEs), offer immense advantages when dealing with complex queries. From simplifying the readability and organization of queries to breaking computations into parts, these clauses have become essential for modern data management.

Here, we will explore the practical applications of SQL WITH clauses in analyzing real estate markets, processing hierarchical structures, and undertaking interactive courses on recursive queries.

Analyzing Real Estate Market in San Francisco Bay Area

The real estate market in the San Francisco Bay Area is a prime example of how SQL WITH clauses can be used. Real estate data of this region is made publicly available by MLS listings; this data is used with CTEs to rank the communities within a city or county.

Here, we will use the CTE to filter the number of houses in a price range and zip code. Consider the houses table in a conventional relational database with numerous columns and features.

We will obtain the number of houses within different zip codes having a specific type and size of bedrooms and is within a price range from the total number of houses in the region. The query will be structured as:

WITH filtered_houses AS (
SELECT zip_code, COUNT(*) as count
FROM houses
WHERE bedrooms = 3 AND price BETWEEN 600000 AND 1000000
GROUP BY zip_code
SELECT zip_code, (count/total_house_in_zip_code)*100 as percentage
FROM filtered_houses
SELECT zip_code, COUNT(*) as total_house_in_zip_code
FROM houses
GROUP BY zip_code
) total_houses_by_zip
ON filtered_houses.zip_code = total_houses_by_zip.zip_code;

This query uses two CTEs. The first CTE, filtered_houses, filters the house based on the price range, type, and size of rooms. The second CTE, total_houses_by_zip, obtains the total number of houses by zip code.

The main query computes the percentage of filtered houses within each zip code by dividing the number of filtered houses by the total number of houses in that zip code. This query demonstrates how SQL WITH clauses can be used to simplify the process of rank-ordering communities by filtering data based on the required parameters.

Processing Hierarchical Structures in Organizations

One of the most significant applications of recursive CTEs is in the processing of hierarchical structures, where the data is organized in a tree-like structure with the nodes of the tree hierarchy representing organizational roles. Companies generate data that represents the workforce hierarchy and its managerial structure.

We will use the employees table to generate the number of employees reporting to the respective manager and its managers manager recursively. Consider the employees table with columns such as employee_id, employee_name, dept_id, and manager_id.

Executors can get data of employees that report to their immediate manager by selecting those whose manager_id equals that managers employee. However, to get all the employees reporting to that manager’s manager, a recursive query is required.

The recursive query starts with the base case, where the query returns all employees with a null manager_id value. The query then recursively adds the employees with their respective managers until the output has no new rows.

Here, we will use the recursive query to get the number of employees directly and indirectly reporting to the CEO.

WITH recursion AS (
SELECT employee_id, manager_id, 1 as levels_of_reporting
FROM employees
WHERE manager_id IS NULL
SELECT e.employee_id, e.manager_id, levels_of_reporting+1
FROM employees e
JOIN recursion ON e.manager_id = recursion.employee_id
SELECT COUNT(*) FROM recursion WHERE levels_of_reporting <= 2;

This query uses a recursive CTE to compute the levels of an org chart and the number of employees reporting to each manager. The base case selects all the employees who report to a manager with null for manager_id.

The recursive case carries out the self-join of the employees table until it has no new joined values. Finally, the query selects those rows where levels_of_reporting is less than or equal to two and counts the number of rows.

SQL Reporting Track

Another practical application of SQL WITH clauses is in creating reports, which are a fundamental part of data management. SQL reports provide valuable insights by making data accessible, allowing users to analyze it in a way that best fits their requirements, and with results visualized as graphs and dashboards.

The SQL Reporting Track enables executors to use SQL clauses to analyze revenue trends and customer behavior with ease. Consider the scenario where an executive wants to analyze the sales trends of his company over a certain period.

With SQL reporting, the executive can get this report in a simplified manner by combining different clauses with simple queries. The data is aggregated based on simple business rules to make the final report easy to read and analyze.

By using SQL WITH clauses and other SQL clauses, the executive can analyze the metadata for reporting purposes. The course on Recursive Queries

The interactive course on recursive queries offers tutorials on recursive CTEs subdivided into simpler concepts.

This course includes 114 coding challenges and sample data to use in the analysis of example scenarios across various industries. It comprises of beginner-level SQL programs and advanced SQL programs, including simple CTEs, nested CTEs, and hierarchical queries.

The course makes use of hands-on coding exercises, which help to impart practical SQL WITH clause knowledge to learners.


SQL WITH clauses have become essential for modern data management, providing legibility and maintainability to complex queries, and enhancing the organization of the computations while computing data. With the examples outlined in this article, executors have a starting point for applying SQL WITH clauses to their data management operations.

The practical applications of SQL WITH clauses are diverse, from analyzing the real estate market to processing hierarchy structures and producing reports. Exploring SQL WITH clauses further is essential for any organization that uses SQL for data manipulation and analysis.

SQL WITH clauses, also known as Common Table Expressions (CTEs), have become essential for modern data management. This article has explored the definition and benefits of SQL WITH clauses, including readability and improved organization, as well as breaking computations into manageable parts and the nesting of computations.

Additionally, it has highlighted practical applications, such as analyzing the real estate market in the San Francisco Bay area and processing hierarchical structures. The article also discussed the usefulness of SQL Reporting Tracks and the course on Recursive Queries.

The practical applications of SQL WITH clauses are diverse and can help organizations make data-driven decisions. By learning and utilizing SQL WITH clauses, data manipulation and analysis tasks can become simpler and more efficient.

Popular Posts