"WITH RECURSIVE": The Unsung Hero of SQL

Let me take you on a journey into the world of SQL, where WITH RECURSIVE is the wizard, and our mission is to organize a recipe book. Sounds exciting, right? Stick around for some learning and tasty metaphors!

The Quest Begins: What is WITH RECURSIVE?

Imagine a world where your boss asks you to generate a sequence of numbers, and you're like, "Where's Excel when I need it?" 🤦‍♂️ But no! You don’t need Excel because SQL gives you a magical tool called WITH RECURSIVE.

It’s like Gandalf of SQL—powerful, versatile, and a little intimidating at first. But once you know how it works, you'll be yelling "You shall not pass!" to unnecessary manual processes.

Understanding WITH RECURSIVE:

Imagine you're a chef making lasagna. You start with a base layer (Step 1), and then you keep adding layers until the dish is ready to serve (Step 3). But here’s the catch: instead of just building the lasagna all at once, you follow a specific pattern.

"WITH RECURSIVE" in SQL is like that lasagna-making process: it's a way to repeatedly add layers until you get to the final result. Here’s how it works:

• Step 1: Start with the base case (the first layer of lasagna):

  • In SQL, you define the first part of the result, which is your starting point. It's like the foundation of your query.

• Step 2: Define the recursive step (adding more layers of lasagna):

  • You then define a recursive step, which essentially says, “Take the last layer and add a new layer on top.” In SQL, this means you are referring back to the previous result to build the next step.

• Step 3: Stop when you reach your goal (when the lasagna is fully baked):

  • This is the point where your query has done enough steps and you get the final result. The SQL query stops when it reaches the end condition.

Let's spice things up with a tasty problem to make WITH RECURSIVE more enjoyable! 🍝

The Challenge: Organizing the Cookbook 📚🍽️

Here’s the problem:
You have a table named "cookbook_titles" with page numbers and recipe names as columns. Your mission is to group recipes so that:

1. Even pages show the recipe title (if available).

2. Odd pages show the next recipe (or nothing if it's blank).

3. Page 0 is empty because it’s the inner cover — like that sad first slice of bread in a loaf. 🥲

Output:
You need a table with three columns:

• left_page_number (even page).

• left_title (recipe title for the even page).

• right_title (recipe title for the next odd page).

(If you're already questioning your life choices after reading the problem summary above, don’t worry —Here’s the detailed question to ease your pain.)

Cooking Up the Solution: 🍲💡

Here’s how we use WITH RECURSIVE to solve above puzzle:

Step 1: Generate Even and Odd Page Numbers

WITH RECURSIVE series AS (
    SELECT 0 AS evens, 1 AS odds
    UNION ALL
    SELECT evens + 2, odds + 2
    FROM series
    WHERE evens + 2 <= (SELECT MAX(page_number) FROM cookbook_titles)
)

• Start with page 0 (even) and page 1 (odd).

• Increment each by 2 until we reach the last page.

This is like building an assembly line for page numbers: one even, one odd, rinse and repeat.

Step 2: Join the Titles with Page Numbers

SELECT 
    s.evens AS left_page_number,
    ct1.title AS left_title,
    ct2.title AS right_title
FROM 
    series s
LEFT JOIN 
    cookbook_titles ct1 ON s.evens = ct1.page_number
LEFT JOIN 
    cookbook_titles ct2 ON s.odds = ct2.page_number;

• Use LEFT JOIN to match even pages with their titles (left_title) and odd pages with the next title (right_title).

• If a page has no recipe, the result is NULL—because not every page gets a recipe. That’s life.

A Quick Look at the Output

Here’s what we get with sample data:

The Takeaway

WITH RECURSIVE is like a Swiss Army knife for SQL—it simplifies complex tasks and makes your life easier. It’s perfect for hierarchical data, sequences, and yes, organizing recipe books!

So next time someone gives you a challenge like this, just remember: You have the power of recursion. And a little humor never hurts. 😉

Happy querying! 🍕