In data warehousing, the snowflake schema is a popular design where the dimension tables are normalized into multiple layers, creating a snowflake-like structure. Snowflake schema offers flexibility and better query performance but can be challenging when it comes to handling recursive queries.

Recursive queries, also known as hierarchical queries, are used to traverse and query data that has a parent-child or hierarchical relationship. In this blog post, we will explore how to work with recursive queries in a Snowflake schema.

Table Structure

Before diving into recursive queries, let’s first define a simple example table structure in Snowflake schema. Imagine we have two dimension tables: category and product. The category table has a self-referencing foreign key parent_id to represent the hierarchical relationship within categories.

CREATE TABLE category (
  id INT PRIMARY KEY,
  name VARCHAR(100),
  parent_id INT
);

CREATE TABLE product (
  id INT PRIMARY KEY,
  name VARCHAR(100),
  category_id INT,
  FOREIGN KEY (category_id) REFERENCES category(id)
);

Recursive Queries with Snowflake

Snowflake provides the WITH RECURSIVE syntax to handle recursive queries. This syntax allows us to define a recursive CTE (Common Table Expression) that iteratively builds the result set based on the initial non-recursive part and the recursive part.

To retrieve all products belonging to a specific category and its subcategories recursively, we can create a recursive CTE as follows:

WITH RECURSIVE category_hierarchy AS (
  SELECT id
  FROM category
  WHERE id = :category_id
  UNION ALL
  SELECT c.id
  FROM category c
  JOIN category_hierarchy ch ON c.parent_id = ch.id
)
SELECT p.id, p.name
FROM product p
JOIN category_hierarchy ch ON p.category_id = ch.id;

In the above example, the initial part of the recursive CTE selects the category with the specified category_id. The recursive part then joins the category_hierarchy CTE to the category table to traverse the hierarchical relationship until all subcategories are included.

Finally, we join the product table with the category_hierarchy CTE to retrieve all products belonging to the specified category and its subcategories.

Conclusion

Working with recursive queries in a Snowflake schema can be achieved using the WITH RECURSIVE syntax. By defining a recursive CTE, you can easily query hierarchical data and traverse the snowflake schema. This offers flexibility and convenience when dealing with complex data relationships.

Remember to optimize your query performance by using appropriate indexes and filtering techniques when working with recursive queries in your Snowflake schema.

#snowflake #recursivequeries