Combine Parent ID and All Child IDs in an Array in a Single Row using Recursive CTE

Are you tired of dealing with hierarchical data and struggling to combine parent IDs with their corresponding child IDs in a single row? Well, you’re in luck! In this article, we’ll explore the magic of recursive Common Table Expressions (CTEs) and show you how to achieve this feat with ease.

What are Recursive CTEs?

Before we dive into the solution, let’s take a step back and understand what recursive CTEs are. A recursive CTE is a type of CTE that allows you to reference itself in the definition, enabling you to recursively query hierarchical data. This powerful feature is supported by most modern databases, including SQL Server, PostgreSQL, and Oracle.

Benefits of Recursive CTEs

So, why should you care about recursive CTEs? Here are some benefits:

• Efficient querying of hierarchical data: Recursive CTEs allow you to query hierarchical data in a single statement, eliminating the need for complex joins or iterative processing.
• Improved performance: By avoiding multiple queries or iteration, recursive CTEs can significantly improve performance, especially for large datasets.
• Simplified code: Recursive CTEs often result in cleaner, more readable code, making maintenance and debugging a breeze.

The Problem: Combining Parent ID and Child IDs in a Single Row

Now, let’s get to the problem at hand. Suppose we have a table called `categories` with the following structure:

ID	Parent ID	Name
1	NULL	Root Category
2	1	Subcategory 1
3	1	Subcategory 2
4	2	Sub-subcategory 1
5	3	Sub-subcategory 2

Our goal is to combine each parent ID with its corresponding child IDs in a single row, like this:

ID	Child IDs
1	[2, 3]
2	[4]
3	[5]

The Solution: Recursive CTE to the Rescue!

Here’s the magic part! Using a recursive CTE, we can achieve our goal in a single statement. Let’s break it down step by step:

Step 1: Define the Anchor Query

The anchor query defines the starting point for our recursive CTE. In this case, we’ll select the parent IDs and their corresponding child IDs:

WITH RECURSIVE category_hierarchy AS (
  SELECT id, parent_id, [id] AS child_ids
  FROM categories
  WHERE parent_id IS NULL  -- anchor query
)

Step 2: Define the Recursive Query

The recursive query joins the categories table with the CTE, selecting the parent ID and concatenating the child IDs:

  SELECT c.id, c.parent_id, h.child_ids + [c.id]
  FROM categories c
  INNER JOIN category_hierarchy h ON c.parent_id = h.id
)

Step 3: Combine the Results

Finally, we’ll use the `STRING_AGG` function (or equivalent) to combine the child IDs into a single array:

SELECT id, STRING_AGG(DISTINCT child_ids, ',') AS child_ids
FROM category_hierarchy
GROUP BY id;

Putting it All Together

Here’s the complete solution:

WITH RECURSIVE category_hierarchy AS (
  SELECT id, parent_id, [id] AS child_ids
  FROM categories
  WHERE parent_id IS NULL  -- anchor query

  UNION ALL

  SELECT c.id, c.parent_id, h.child_ids + [c.id]
  FROM categories c
  INNER JOIN category_hierarchy h ON c.parent_id = h.id
)
SELECT id, STRING_AGG(DISTINCT child_ids, ',') AS child_ids
FROM category_hierarchy
GROUP BY id;

Run this query against your `categories` table, and voilà! You should see the desired output, with each parent ID combined with its corresponding child IDs in a single row.

Conclusion

In this article, we’ve demonstrated the power of recursive CTEs in combining parent IDs with their corresponding child IDs in a single row. By breaking down the problem into smaller, manageable pieces, we’ve shown how to write a clear, efficient, and maintainable solution using recursive CTEs.

Remember, recursive CTEs are a powerful tool in your SQL toolkit, and with practice, you can tackle even the most complex hierarchical data challenges.

Thanks for reading, and happy coding!

Frequently Asked Questions

Get the answers to your burning questions about combining parent id and all child ids in an array in a single row using recursive CTE!