Recursive CTEs: What They Are and How to Use Them

Recursive Common Table Expressions (CTEs) are a powerful tool for querying hierarchical data. CTEs allow you to define a recursive query that can be used to generate a set of results that is based on the results of previous iterations of the query.

Recursive CTEs are often used to query data that is organized in a tree-like structure. For example, you could use a recursive CTE to query an organization chart, a family tree, or a directory of websites.

How do recursive CTEs work?

A recursive CTE consists of three parts:

1. The anchor query: The anchor query is the initial query that is executed. The anchor query typically defines the starting point for the recursion.
2. The recursive query: The recursive query is the query that is executed repeatedly. The recursive query typically uses the results of the anchor query to generate the results of the next iteration.
3. The termination condition: The termination condition is the condition that determines when the recursion should stop. The termination condition is typically based on the results of the recursive query.

Here is an example of a recursive CTE:

WITH RECURSIVE employees AS (
  -- Anchor query
  SELECT employee_id, first_name, last_name
  FROM employees
  WHERE manager_id IS NULL
  -- Recursive query
  UNION ALL
  -- Recursive term
  SELECT e.employee_id, e.first_name, e.last_name
  FROM employees e
  JOIN employees m ON e.manager_id = m.employee_id
)
SELECT *
FROM employees;

This CTE defines a recursive query that can be used to query an organization chart. The anchor query selects all employees who do not have a manager. The recursive query then joins the anchor query to the employees table to find all employees who are managed by the employees in the anchor query. The recursion continues until all employees in the organization chart have been included in the results.

When should you use recursive CTEs?

Recursive CTEs should be used when you need to query hierarchical data. Recursive CTEs are a powerful tool for generating a set of results that is based on the results of previous iterations of the query.

Recursive CTEs should not be used when you can achieve the same results with a non-recursive query. Non-recursive queries are typically easier to understand and maintain.

Conclusion

Recursive CTEs are a powerful tool for querying hierarchical data. CTEs allow you to define a recursive query that can be used to generate a set of results that is based on the results of previous iterations of the query.

Recursive CTEs should be used when you need to query hierarchical data. Recursive CTEs should not be used when you can achieve the same results with a non-recursive query.

If you are interested in learning more about recursive CTEs, there are many resources available online and in libraries. You can also take courses on SQL and data science at universities and online learning platforms.

Google Bard is the copilot writing this blog.