Hi there and welcome to this comprehensive guide on SQL Server Recursive CTE. If you are looking to master this advanced SQL Server concept, you have come to the right place. Recursive CTE, or Common Table Expressions, is a powerful tool that allows you to perform complex recursive queries on SQL Server databases. In this article, we will take you through everything you need to know about recursive CTE, from the basics to advanced techniques. We will cover a wide range of topics, including the syntax, use cases, and best practices. If you are ready to take your SQL Server skills to the next level, let’s get started!

Table of Contents

1. Introduction
2. What is Recursive CTE?
3. The Syntax of Recursive CTE
4. Basic Recursive CTE
5. Recursive CTE with Multiple CTEs
6. Recursive CTE with Aggregation
7. Recursive CTE with Joins
8. Advanced Techniques
9. Performance Tuning
10. Best Practices
11. FAQs

Introduction

Structured Query Language (SQL) is the standard programming language for managing and manipulating relational databases. SQL Server, one of the most popular relational database management systems (RDBMS), supports a rich set of SQL features that enable developers to write efficient and complex queries. One of these features is Common Table Expressions (CTEs).

CTEs are temporary named result sets that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. They are especially useful for creating recursive queries. A recursive query is a type of query that refers to the result of the same query multiple times in order to derive the final result. Recursive queries are commonly used in hierarchical data structures, such as organizational charts or bill-of-materials data, where each row has a parent-child relationship with one or more other rows.

Recursive CTE is a powerful tool that simplifies the process of writing recursive queries in SQL Server. With recursive CTE, you can write queries that are concise, easy to read, and efficient. Recursive CTE allows you to define a recursive relationship between two or more CTEs, making it possible to traverse hierarchical data structures in a natural and intuitive way.

What is Recursive CTE?

A recursive CTE is a CTE that references itself in the SELECT statement. A recursive CTE consists of two parts:

• The anchor member: The first part of the recursive CTE, which returns the initial set of rows to be processed. The anchor member is a regular SELECT statement that defines the base case of the recursion.
• The recursive member: The second part of the recursive CTE, which references the CTE itself in the SELECT statement. The recursive member is a SELECT statement that defines the recursive step of the recursion.

When a recursive CTE is executed, the anchor member is executed first, returning the initial set of rows to be processed. The recursive member is then executed repeatedly until no more rows are returned. Each iteration of the recursive member builds on the results of the previous iteration, until the final result set is obtained.

The Syntax of Recursive CTE

The syntax of recursive CTE in SQL Server is as follows:

WITH CTE_name (column1, column2, ..., columnN)
AS
(
  -- Anchor member
  SELECT ...
  UNION ALL
  -- Recursive member
  SELECT ...
  FROM CTE_name
  WHERE ...
)
SELECT ...
FROM CTE_name
WHERE ...

In this syntax, CTE_name is the name of the recursive CTE, and column1, column2, ..., columnN are the names of the columns returned by the SELECT statements in the anchor and recursive members.

The anchor member is a regular SELECT statement that defines the base case of the recursion. It must not reference the CTE_name table alias.

The recursive member is a SELECT statement that defines the recursive step of the recursion. It must reference the CTE_name table alias and return the same number and type of columns as the anchor member.

The UNION ALL operator is used to combine the results of the anchor and recursive members. The WHERE clause in the recursive member is used to terminate the recursion by returning an empty result set.

Basic Recursive CTE

Let’s start with a simple example of a recursive CTE. Suppose you have a table called Employees that contains information about the employees in your organization. Each employee belongs to a department, and each department has a manager. The Employees table has the following columns:

Column Name	Data Type	Description
EmployeeID	int	The unique identifier of the employee.
FirstName	nvarchar(50)	The first name of the employee.
LastName	nvarchar(50)	The last name of the employee.
DepartmentID	int	The unique identifier of the department to which the employee belongs.
ManagerID	int	The unique identifier of the employee’s manager.

In this example, we want to create a recursive CTE that returns all the employees who report directly or indirectly to a given manager. Here is how we can do it:

WITH EmployeeHierarchy (EmployeeID, FirstName, LastName, DepartmentID, ManagerID, Level)
AS
(
  -- Anchor member
  SELECT EmployeeID, FirstName, LastName, DepartmentID, ManagerID, 0
  FROM Employees
  WHERE ManagerID = @ManagerID
  UNION ALL
  -- Recursive member
  SELECT e.EmployeeID, e.FirstName, e.LastName, e.DepartmentID, e.ManagerID, Level + 1
  FROM Employees e
  INNER JOIN EmployeeHierarchy eh
  ON e.ManagerID = eh.EmployeeID
  WHERE e.ManagerID <> e.EmployeeID
)
SELECT EmployeeID, FirstName, LastName, DepartmentID, ManagerID, Level
FROM EmployeeHierarchy
ORDER BY Level, LastName, FirstName

Let’s go through this code step by step:

• We start by defining a recursive CTE called EmployeeHierarchy.
• The anchor member returns all the employees who report directly to the given manager. We use a WHERE clause to filter the employees based on their ManagerID.
• The recursive member joins the Employees table with the previous iteration of the EmployeeHierarchy CTE. We use an INNER JOIN to ensure that we only retrieve employees who report directly or indirectly to the given manager. We also add 1 to the Level column to indicate the depth of each employee in the hierarchy.
• We select all the columns from the EmployeeHierarchy CTE and order the results by the Level column, followed by the last name and first name of each employee.

The result of this query will be a table that lists all the employees who report directly or indirectly to the given manager, along with their department and level in the hierarchy. Here is a sample result:

EmployeeID	FirstName	LastName	DepartmentID	ManagerID	Level
3	Mike	Scott	2	1	1
4	Emily	Thomas	2	1	1
5	John	Lee	3	2	2
6	Alice	Tan	3	2	2

This result shows that there are two employees who report directly to the given manager (Mike Scott and Emily Thomas), and two employees who report indirectly to the given manager (John Lee and Alice Tan).

Recursive CTE with Multiple CTEs

Recursive CTE can be combined with other CTEs to create more complex queries. Here is an example of a recursive CTE that uses two CTEs to compute the Fibonacci sequence:

WITH Fibonacci (N, F1, F2)
AS
(
  -- Anchor member
  SELECT 1, 0, 1
  UNION ALL
  -- Recursive member
  SELECT N + 1, F2, F1 + F2
  FROM Fibonacci
  WHERE N <= @N
),
FibonacciSequence (N, F)
AS
(
  SELECT N, F1
  FROM Fibonacci
  WHERE N > 1
)
SELECT N, F
FROM FibonacciSequence
ORDER BY N

In this example, we define two CTEs:

• Fibonacci: This CTE defines the recursive relationship between the Fibonacci numbers. The anchor member returns the first two numbers in the sequence (0 and 1), while the recursive member computes the subsequent numbers by adding the two previous numbers together. The N column keeps track of the sequence number of each Fibonacci number.
• FibonacciSequence: This CTE filters and selects the final Fibonacci sequence. We filter out the first number (0) and select only the N and F1 columns from the Fibonacci CTE.

The result of this query will be a table that lists the first N Fibonacci numbers, along with their sequence number. Here is a sample result:

N	F
2	1
3	1
4	2
5	3
6	5
7	8
8	13
9	21
10	34

This result shows the first 10 Fibonacci numbers, starting from 1.

Recursive CTE with Aggregation

Recursive CTE can also be used to perform aggregation on hierarchical data. Here is an example:

WITH CategoryHierarchy (CategoryID, CategoryName, ParentCategoryID, TotalProducts)
AS
(
  -- Anchor member
  SELECT CategoryID, CategoryName, ParentCategoryID, COUNT(*)
  FROM Categories
  WHERE ParentCategoryID IS NULL
  GROUP BY CategoryID, CategoryName, ParentCategoryID
  UNION ALL
  -- Recursive member
  SELECT c.CategoryID, c.CategoryName, c.ParentCategoryID, COUNT(*)
  FROM Categories c
  INNER JOIN CategoryHierarchy ch
  ON c.ParentCategoryID = ch.CategoryID
  GROUP BY c.CategoryID, c.CategoryName, c.ParentCategoryID
)
SELECT CategoryID, CategoryName, ParentCategoryID, TotalProducts
FROM CategoryHierarchy
ORDER BY CategoryName

In this example, we have a table called Categories that contains information about the categories of products in our online store. Each category may have one or more subcategories, forming a hierarchical structure. We want to use recursive CTE to compute the total number of products in each category and its subcategories.

We start by defining a recursive CTE called CategoryHierarchy. The anchor member returns all the top-level categories (i.e., the categories that have no parent category). We use a GROUP BY clause to compute the total number of products in each top-level category.

The recursive member joins the Categories table with the previous iteration of the CategoryHierarchy CTE. We use an INNER JOIN to ensure that we only retrieve subcategories that belong to the current iteration of the hierarchy. We also use a GROUP BY clause to compute the total number of products in each subcategory.

The result of this query will be a table that lists all the categories and their total number of products, including their subcategories. Here is a sample result:

CategoryID	CategoryName	ParentCategoryID	TotalProducts
1	Books	NULL	120
2	Novels	1	50
3	Comics	1	70
4	Marvel	3	30
5	DC	3	40

This result shows that the top-level category “Books” has 120 products, including 50 novels and 70 comics. The “Comics” category, in turn, has 30 Marvel comics and 40 DC comics.

Recursive CTE with Joins

Recursive CTE

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