How Do Recursive CTEs Work Step-by-Step?

Recursive CTE Execution Breakdown

Recursive CTEs follow a step-by-step execution process that ultimately yields the desired result set. Let's break down each step line by line for a better understanding.

WITH abcd AS (

This line declares the recursive CTE named "abcd." It serves as a temporary table that accumulates the results.

-- anchor

The first select statement acts as the anchor, selecting rows that meet the anchor condition. In this case, it fetches rows from @tbl with null ParentID. This results in the initial population of the "abcd" CTE.

SELECT id, Name, ParentID, CAST(Name AS VARCHAR(1000)) AS Path

This line extracts the ID, Name, ParentID, and a calculated column "Path" that initially contains only the row's Name.

FROM @tbl

The data comes from the temporary table @tbl.

WHERE ParentId IS NULL

This condition filters rows where the ParentID is null, effectively selecting the top-level rows.

UNION ALL

This operator combines the anchor select statement with the recursive member.

--recursive member

The second select statement forms the recursive member, which performs iterations based on the anchor.

SELECT t.id, t.Name, t.ParentID, CAST((a.path '/' t.Name) AS VARCHAR(1000)) AS "Path"

This line retrieves ID, Name, ParentID, and a modified column "Path" that concatenates the parent's "Path" with the current row's Name.

FROM @tbl AS t

The data comes from the temporary table @tbl, aliased as "t."

JOIN abcd AS a

This join links rows in "t" with those in the existing "abcd" CTE using the ParentID column.

ON t.ParentId = a.id

The join condition ensures that child rows are matched with their parent rows.

SELECT * FROM abcd

Finally, this statement returns the full result set from the recursive CTE, including all iterations.

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