Rewrite Recursive CTE using While Loop

Recursive Common Table Expressions (CTEs) are a powerful feature in SQL Server that allow you to write complex queries in a more manageable way. However, they can also lead to performance issues and maximum recursion limits when dealing with deeply nested relationships between tables. In this article, we will explore an alternative approach to recursive CTEs using while loops.

Understanding Recursive CTEs

Before diving into the while loop implementation, let’s quickly review how recursive CTEs work in SQL Server. A recursive CTE is a temporary result set that is defined within a SELECT statement and can be referenced by itself. The basic syntax of a recursive CTE is as follows:

WITH RECURSIVE CTE AS (
    -- anchor query: selects the starting points for the recursion
    ...
    UNION ALL
    -- recursive query: refers to the previous CTE and appends new rows
    SELECT T.*
    FROM #CTE AS C
    INNER JOIN TABLE1 T ON C.ID = T.ID
)
SELECT *
FROM CTE;

In our sample query, we use two recursive steps:

Anchor Query: SELECT DISTINCT ... FROM TABLE1 selects the initial distinct values from TABLE1.
Recursive Step: The outer query joins the current CTE with TABLE1, filters out rows where the item type is ‘individual’, and appends new results.

Understanding Performance Issues

Recursive CTEs are efficient because they re-use memory for each recursive iteration, but there are performance issues to consider:

Maximum Recursion Limit: SQL Server has a maximum recursion limit (100 by default). If you exceed this limit, your query will timeout or terminate unexpectedly.
Query Performance: Recursive CTEs can lead to poor performance due to the overhead of repeated joins and table scans.

Rewriting Recursive CTE with While Loop

Now that we’ve explored recursive CTEs, let’s dive into rewriting them using a while loop. Here is our rewritten query:

DECLARE @LoopCount INT = 0;

SELECT DISTINCT
        ID,
        [NAME],
        ID AS PARENTID,
        ITEMTYPE,
        COST
INTO #CTE
FROM TABLE1

WHILE @@ROWCOUNT > 0 AND @LoopCount < 100
BEGIN
    INSERT INTO #CTE
    SELECT T.ID,
           C.[NAME],
           C.ID AS PARENTID,
           T.ITEMTYPE,
           T.COST
    FROM #CTE AS C
         INNER JOIN TABLE1 T ON C.ID = T.ID
    WHERE C.ITEMTYPE <> 'individual';

    SET @LoopCount += 1;
END;

IF @LoopCount = 100
    THROW 50000, 'Max recursion limit exceeded', 0

SELECT *
FROM #CTE;

Let’s break down the main components of this rewritten query:

Anchor Query

The anchor query is similar to that in our recursive CTE example. We select distinct values from TABLE1 into a temporary result set.

DECLARE @LoopCount INT = 0;

SELECT DISTINCT
        ID,
        [NAME],
        ID AS PARENTID,
        ITEMTYPE,
        COST
INTO #CTE
FROM TABLE1;

While Loop

The while loop is the heart of our rewritten query. We use a loop counter variable, @LoopCount, to control the number of iterations.

WHILE @@ROWCOUNT > 0 AND @LoopCount < 100
BEGIN
    -- loop body here
END;

Inside the loop, we insert new rows into our temporary result set (#CTE).

INSERT INTO #CTE
SELECT T.ID,
       C.[NAME],
       C.ID AS PARENTID,
       T.ITEMTYPE,
       T.COST
FROM #CTE AS C
      INNER JOIN TABLE1 T ON C.ID = T.ID
WHERE C.ITEMTYPE <> 'individual';

We also increment our loop counter variable and check if we’ve reached the maximum recursion limit. If exceeded, we throw an error.

SET @LoopCount += 1;

IF @LoopCount = 100
    THROW 50000, 'Max recursion limit exceeded', 0;

Loop Body

The loop body is similar to our recursive step in the CTE example:

We join the current temporary result set (#CTE) with TABLE1.
We filter out rows where the item type is ‘individual’.
We insert new results into our temporary result set.

INSERT INTO #CTE
SELECT T.ID,
       C.[NAME],
       C.ID AS PARENTID,
       T.ITEMTYPE,
       T.COST
FROM #CTE AS C
      INNER JOIN TABLE1 T ON C.ID = T.ID
WHERE C.ITEMTYPE <> 'individual';

Key Differences between Recursive CTEs and While Loops

There are a few key differences between recursive CTEs and while loops:

Loop Control: In a recursive CTE, the loop is controlled by SQL Server’s built-in recursion mechanism. With a while loop, you have complete control over when to stop iterating.
Performance: Recursive CTEs can be more efficient because they re-use memory for each iteration. While loops, on the other hand, require storing intermediate results in a temporary result set (#CTE).
Flexibility: While loops offer greater flexibility than recursive CTEs because you can iterate any number of times and control loop flow explicitly.

Conclusion

Recursive Common Table Expressions (CTEs) are powerful but sometimes lead to performance issues or exceed SQL Server’s maximum recursion limit. In this article, we explored an alternative approach using while loops. While loops provide more control over iteration and offer greater flexibility than recursive CTEs, they can also be less efficient due to the overhead of repeated joins and table scans.

When deciding between a recursive CTE and a while loop, consider your specific use case:

Use recursive CTEs for complex queries with deeply nested relationships.
Use while loops when you need greater control over iteration or want to avoid SQL Server’s recursion limit.

We hope this article provided valuable insights into rewriting recursive CTEs using while loops. With these techniques under your belt, you’ll be better equipped to tackle complex queries and optimize performance in SQL Server.

Last modified on 2025-04-13