Recursive CTE Employee Hierarchy Query – Manager Sql

You are an expert SQL developer tasked with writing a production‑ready recursive Common Table Expression (CTE) that extracts an employee hierarchy from a single “Employees” table. Follow the steps and deliverables exactly as described below.

Task Overview Write a recursive CTE query that, given a manager’s employee ID, returns every subordinate in the reporting chain, including the manager themselves, with the following columns for each row:

1. EmployeeID – the employee’s unique identifier.
2. EmployeeName – the employee’s full name.
3. ManagerID – the direct manager’s employee ID (NULL for the top‑level manager).
4. HierarchyLevel – integer depth where the starting manager is level 1, their direct reports level 2, etc.
5. Path – a string representing the chain of IDs from the starting manager to the employee, separated by “→” (e.g., 1→4→7).
6. IsCycle – a Boolean flag indicating whether the row is part of a cycle (should be 0 for normal rows, 1 for rows that would create a cycle).

The query must:

• Use a parameterizable starting manager ID (e.g., @StartManagerID).
• Prevent infinite loops by detecting cycles and marking them with IsCycle = 1.
• Order the final result set first by HierarchyLevel ascending, then by Path alphabetically.
• Be compatible with SQL Server 2019 (or later) syntax, but also include a brief note on any changes needed for PostgreSQL or MySQL 8+.

Schema (provided for reference)

CREATE TABLE Employees (
    EmployeeID   INT PRIMARY KEY,
    EmployeeName NVARCHAR(100) NOT NULL,
    ManagerID    INT NULL,
    CONSTRAINT FK_Employees_Manager FOREIGN KEY (ManagerID) REFERENCES Employees(EmployeeID)
);

Assume the table is already populated with realistic data, and that occasional data errors may create cycles (e.g., Employee 5 reports to Employee 9, and Employee 9 reports back to Employee 5).

Deliverables

1. Complete SQL script (enclosed in a markdown code fence with language identifier sql) that:

   • Declares the @StartManagerID variable (use INT type) and sets it to a placeholder value (e.g., /* TODO: set start manager ID */).
   • Implements the recursive CTE meeting all requirements.
   • Includes a final SELECT statement that returns the columns listed above.

2. Explanation (plain text, numbered list) that:

   • Describes how the CTE works, especially the cycle‑detection logic.
   • Shows how HierarchyLevel and Path are built.
   • Highlights any performance considerations (e.g., indexing recommendations).

3. Sample output (markdown table) for a hypothetical hierarchy where the starting manager ID is 1. Use at least five rows, including one row that demonstrates a cycle (IsCycle = 1).

4. Portability notes (brief bullet points) indicating what must be changed for PostgreSQL and MySQL 8+ (e.g., variable declaration, string concatenation operator).

Formatting Requirements

• The SQL script must be a single, copy‑and‑pasteable block.
• The explanation must be a numbered list (1., 2., …).
• The sample output must be a markdown table with headers matching the column names.
• The portability notes must be a bulleted list.

Please provide the complete response adhering strictly to the structure above. No additional commentary or filler text is needed.