Have you ever wondered how recursion can be implemented in SQL Server? In this blog post, we will explore the concept of recursive Common Table Expressions (CTEs) and how they can be used to solve complex problems.

Recursion is a powerful technique in computer science that allows a function or query to call itself repeatedly until a certain condition is met. It can be used to solve problems that involve hierarchical or self-referencing data structures. In SQL Server, we can achieve recursion using CTEs.

Let’s start with a simple example. Imagine you are given a choice between receiving $1,000,000 at the end of the month or starting with a penny on day 1 and having the amount doubled every day for the entire month. Which option would you choose?

To solve this problem using recursion, we can create a recursive CTE. A CTE consists of two parts: the anchor member and the recursive member. These two parts are connected using the UNION ALL operator.

Let’s take a look at the code:

WITH myWealth (d, v) AS (
    -- anchor member, day 1
    SELECT 1, CAST(0.01 AS numeric(38,2))
    UNION ALL
    -- recursive member, double the value for the next day
    SELECT myWealth.d + 1, myWealth.v * 2
    FROM myWealth
    WHERE myWealth.d <= 31
)
SELECT 'day' = myWealth.d, 'payment' = myWealth.v, 'lump sum' = 1000000,
    'decision' = CASE WHEN myWealth.v < 1000000 THEN 'Good Decision' ELSE 'Bad decision' END
FROM myWealth;

In this code, the anchor member represents the base case or starting point of the recursion. It initializes the day (represented by ‘d’) as 1 and the payment amount (represented by ‘v’) as 0.01. The recursive member then queries the CTE itself, incrementing the day by 1 and doubling the payment amount for each subsequent day. The recursion stops when the day reaches 31.

When we run this code, we get the results that show the payment amount for each day and whether choosing the penny option was a good decision or not.

Recursion can be a powerful tool in SQL Server, allowing us to solve complex problems that involve hierarchical or self-referencing data. It is important to understand the concept of recursive CTEs and how they can be used effectively in your SQL queries.

So, the next time you come across a problem that requires recursion, remember that SQL Server has the capability to handle it using CTEs.

Happy coding!