How Does a Recursive CTE Execute Its Nested UNION ALL Operations Step-by-Step?

How a Recursive CTE Runs, Line by Line

Recursive CTEs, commonly referred to as hierarchical queries, are used to traverse and extract information from tree-like data structures. Understanding how these queries execute can be challenging.

A recursive CTE operates as a series of nested UNION ALL operations, resembling the following:

WITH abcd1 AS
(
    SELECT *
    FROM @tbl
    WHERE ParentID IS NULL
)
UNION ALL
WITH abcd2 AS
(
    SELECT t.*
    FROM abcd1
    JOIN @tbl t
    ON t.ParentID = abcd1.id
)
UNION ALL
WITH abcd3 AS
(
    SELECT t.*
    FROM abcd2
    JOIN @tbl t
    ON t.ParentID = abcd2.id
)
...

Each iteration of the recursive member block creates a new result set by joining the previous iteration with the base table. This process continues until the stopping condition is met, determined by the absence of rows returned.

In the example provided:

1. Anchor (SELECT ...):

   • Retrieves records from the @tbl table with a null ParentID, representing the root of the hierarchy.

2. Recursive Member (SELECT ...):

   • Joins the current iteration with the base table based on the ParentID column, effectively traversing the hierarchy.

3. Path Calculation:

   • Concatenates the Path column from the previous iteration with the current Name, building the hierarchical path.

4. Iteration (UNION ALL):

   • Combines the results of the anchor and recursive member blocks, creating a new result set for the next iteration.

5. Selection (SELECT * FROM abcd):

   • Displays the final result set, which includes all nodes and their respective hierarchical paths.

It's important to note that the anchor is not skipped in subsequent iterations. Instead, it acts as the starting point for each recursive member to explore the hierarchy. To prevent duplicates in the final result, a unique identifier (Path) is used to differentiate between records with the same name that exist at different levels within the hierarchy.

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