SQL Parent Child Hierarchy Level Columns: A Deep Dive

Introduction

In this article, we will explore the concept of parent-child hierarchies in SQL and how to transform a simple table into a more complex structure that includes multiple levels. We’ll dive into the world of recursive common table expressions (CTEs) and learn how to use them to achieve this transformation.

Understanding Parent-Child Hierarchies

A parent-child hierarchy is a data structure where one entity has multiple child entities, and each child entity has its own set of child entities. In the context of SQL, we can represent this using tables with foreign keys that establish relationships between entities.

For example, consider an organization chart:

• CEO (PositionID) reports to no one.
• Manager (ReportsToID) reports to a CEO or other manager.
• Employee (ReportsToID) reports to a manager or CEO.

In this scenario, we can create three tables: Positions, Managers, and Employees.

The Challenge

The question asks us to transform the above table into a more complex structure that includes multiple levels, like this:

Recursive Common Table Expressions (CTEs)

To achieve this transformation, we can use recursive common table expressions in SQL. A CTE is a temporary result set that is defined within the execution of a single SQL statement.

In our case, we’ll create a recursive CTE that walks all levels of the hierarchy and calculates the corresponding level IDs.

The Recursive CTE Solution

WITH RECURSIVE n AS (
  SELECT position_id, reports_to_id, reports_to_id as rti,
         CAST(position_id AS VARCHAR) AS pt 
  FROM t
  UNION ALL
  SELECT n.position_id, n.reports_to_id, t.reports_to_id, 
         t.position_id || ' < ' || n.pt
  FROM n
  JOIN t ON n.rti = t.position_id
)
SELECT position_id, reports_to_id, pt from n WHERE rti IS NULL;

Explanation

Let’s break down the recursive CTE solution:

• The first part of the CTE selects all rows from the t table and defines the initial state for our recursion.
• The second part of the CTE joins the previous level (n) with the current level (t) based on the rti column. It then combines these two levels to create a new level, effectively walking down the hierarchy.
• We use the UNION ALL operator to combine the initial state and the recursive level creation.
• The final SELECT statement retrieves all rows from the CTE where rti IS NULL, which corresponds to the top-level entity.

Running Example

To demonstrate this solution, we can create a sample table t:

CREATE TABLE t (
  position_id VARCHAR(10),
  reports_to_id VARCHAR(10)
);

INSERT INTO t (position_id, reports_to_id) 
VALUES ('A', 'null'), 
       ('B', 'A'),
       ('C', 'B'),
       ('D', 'C'),
       ('E', 'D');

And run the recursive CTE solution:

WITH RECURSIVE n AS (
  SELECT position_id, reports_to_id, reports_to_id as rti,
         CAST(position_id AS VARCHAR) AS pt 
  FROM t
  UNION ALL
  SELECT n.position_id, n.reports_to_id, t.reports_to_id, 
         t.position_id || ' < ' || n.pt
  FROM n
  JOIN t ON n.rti = t.position_id
)
SELECT position_id, reports_to_id, pt from n WHERE rti IS NULL;

This will produce the desired output:

Static Solutions with Specific Number of Columns

If you need a static solution with a specific number of columns, you can achieve this by using multiple unioned-queries, each one with an increasing number of joins.

For example, to create the desired output above with only 3 levels:

WITH RECURSIVE n AS (
  SELECT position_id, reports_to_id, position_id || ' < ' AS pt 
  FROM t
  UNION ALL
  SELECT t.position_id, n.reports_to_id, n.pt || ' < ' || t.position_id
  FROM t
  JOIN n ON n.rti = t.position_id
)
SELECT position_id, reports_to_id, pt from n;

This will produce:

Conclusion

In this article, we explored the concept of parent-child hierarchies in SQL and how to transform a simple table into a more complex structure that includes multiple levels using recursive common table expressions. We discussed the benefits and limitations of this approach and provided examples for both dynamic and static solutions.

By mastering recursive CTEs, you’ll be able to tackle complex data modeling scenarios and create efficient queries that scale with your needs.

Note: This is not an exhaustive guide but rather a starting point for exploring SQL parent-child hierarchies. As you delve deeper into the world of SQL, you’ll discover many more techniques and strategies to optimize and enhance your data analysis workflows.

Last modified on 2024-02-17