What are common table expressions (CTEs) and how do you use them in PostgreSQL?

Q: What are common table expressions (CTEs) and how do you use them in PostgreSQL?

Learn the answer to "What are common table expressions (CTEs) and how do you use them in PostgreSQL?" with detailed explanations, code examples, and best practices on DeployU.

The Scenario

You are a data analyst at an e-commerce company. You are trying to write a complex query that involves multiple levels of subqueries. The query is difficult to read and maintain.

The Challenge

Explain what common table expressions (CTEs) are in PostgreSQL and how you would use them to solve this problem. What are the key benefits of using CTEs?

Wrong Approach

A junior engineer might not be aware of CTEs. They might try to solve this problem by using a series of subqueries, which would be difficult to read and maintain.

Addresses symptoms, not root cause

Right Approach

A senior engineer would know that CTEs are the perfect tool for this job. They would be able to explain what CTEs are and how to use them to write concise and readable queries for a variety of different use cases.

Step 1: Understand What CTEs Are

A common table expression (CTE) is a temporary named result set that you can reference within another SQL statement.

Step 2: The `WITH` Clause

A CTE is defined using the WITH clause.

WITH my_cte AS (
    SELECT * FROM my_table
)
SELECT * FROM my_cte;

Step 3: Solve the Problem

Here’s how we can use a CTE to simplify a complex query:

Without CTEs:

SELECT *
FROM (
    SELECT *
    FROM my_table
    WHERE my_column = 'my_value'
) as my_subquery
WHERE my_subquery.my_other_column = 'my_other_value';

With CTEs:

WITH my_cte AS (
    SELECT *
    FROM my_table
    WHERE my_column = 'my_value'
)
SELECT *
FROM my_cte
WHERE my_other_column = 'my_other_value';

As you can see, the query with the CTE is much more readable and easier to understand.

Recursive CTEs

You can also use recursive CTEs to perform recursive queries, such as traversing a hierarchical data structure.

Systematic, production-ready debugging

Practice Question

You want to write a query to find all the employees who report to a specific manager, and all the employees who report to those employees, and so on. Which of the following would you use?

Questions