In SQL Server, how can you create a recursive query that references the results of the previous step, for example to traverse a hierarchy or organizational tree?

Introduction / Context:
Many databases need to represent hierarchical data such as employee reporting structures, folder trees, or bill of materials. SQL Server supports recursive queries that can walk these hierarchies without resorting to procedural loops. The standard technique uses recursive common table expressions, or recursive CTEs. Interviewers often test whether candidates know the basic structure of such queries.

Given Data / Assumptions:
We are using Microsoft SQL Server and Transact SQL.Hierarchical data is stored in a table where each row may reference a parent row, for example with a ParentId column.The goal is to write a query that returns multiple levels of the hierarchy.We focus on the logical pattern, not on full code for every scenario.

Concept / Approach:
A recursive common table expression consists of an anchor member and a recursive member. The anchor member selects the starting rows of the hierarchy, such as the root node or a specific employee. The recursive member joins the CTE back to the base table using a relationship like parent to child and is combined with the anchor using UNION ALL. SQL Server executes the anchor first, then repeatedly applies the recursive member until no new rows are produced. The final SELECT statement reads from the CTE, returning the full hierarchical result.

Step-by-Step Solution:
Step 1: Recall that the syntax starts with WITH CTEName AS, followed by a query block.Step 2: Define the anchor member that selects the starting point of the hierarchy.Step 3: Define the recursive member that references the CTE itself and joins to the base table on parent child relationships.Step 4: Combine the anchor and recursive member with UNION ALL inside the CTE definition.Step 5: Select from the CTE to get the final recursive result, which matches the description in option A.

Verification / Alternative check:
A simple example is a table Employees with EmployeeId and ManagerId columns. You can define a CTE that first selects the top manager in the anchor part and then joins the CTE to Employees on ManagerId equals EmployeeId in the recursive part. Running this query returns all subordinates at every level. This example shows that recursive CTEs implement recursion using the WITH clause and UNION ALL, confirming that option A describes the correct approach.

Why Other Options Are Wrong:
Option B suggests using a cursor that prints values without a SELECT. While cursors can iterate through rows, they do not represent the declarative recursive query pattern asked in the question. Option C describes a simple SELECT that cannot traverse multiple hierarchy levels on its own. Option D mentions a CHECK constraint, which is used to enforce data validation, not to implement recursive queries. Option E claims that a database option can make all queries recursive automatically, which is not a real feature in SQL Server.

Common Pitfalls:
Developers sometimes forget to include a termination condition in the recursive member, which can lead to errors if cycles exist in the data. Another pitfall is using UNION instead of UNION ALL, which can add unnecessary overhead and possibly remove legitimate duplicate rows. Performance tuning may involve indexing the parent child columns to speed up joins. Understanding the basic pattern of recursive CTEs is a strong foundation for solving hierarchy related problems.

Final Answer:
The correct answer is: By using a recursive common table expression defined with WITH, an anchor query, and a recursive member combined with UNION ALL.