Advanced SQL Interview Topic : What is Recursive CTE in SQL ?

A Recursive CTE is just like a CTE but it references to itself again and again unless it reaches to a termination check or returns all possible result. Due to its repeatedly referencing to itself the word Recursive is used.

Let us understand Recursive CTE so that you will be able to use it and you can easily answer in. Interview

Recursive CTE in SQL

A Recursive CTE references to itself , it will return the result subset and repeatedly references itself and stops when it returns all results.

Basically in laymen term a Recursive CTE will reference to itself as per our given condition and it will terminate on a particular condition which is defined by us .

Recursive CTE Syntax

Before understanding about the use case of Recursive CTE let us first understand about the Syntax of Recursive CTE.

A Recursive CTE has 3 important elements:

1. Anchor Query: This is the initial query that provides the starting point of the recursion.
2. Recursive Query: This part references the CTE itself and continues the recursion based on the result of the anchor member.
3. Termination Check: Recursion continues until a termination condition is met, such as a specific number of iterations or when no more rows are returned

** Anchor Query and Recursive Query are connected through Union All so it combines all the results.**

Steps to Write A Recursive CTE query in SQL

1. Use the syntax just like CTE but use word Recursive before it.
2. Now inside the brackets first write a base query. ( You can make reference to the table from which you want result. )
3. No make use of Union All ( It will connect your anchor query with your recursive query. )
4. Now write another query and it will use reference from your Recursive CTE name you have given
5. Give a termination condition where it recursion of this query shall end.
6. Now just like CTE call data from your created Recursive CTE.

Related – Important SQL Topic’s for Data Analyst Interview

Difference between Recursive and non Recursive CTE in SQL.

Lets us understand what is the difference between CTE and Recursive CTE.

CTE (Common Table Expression):

• A temporary result set used to simplify complex queries.
• No recursion; it’s evaluated once and referenced multiple times.

Recursive CTE:

• Used for hierarchical data (e.g., org charts) and iterative tasks.
• A special CTE that references itself, allowing for recursion.
• Consists of a base query (anchor) and a recursive query.

Key difference is that a CTE is non-recursive and used for simplifying queries, while Recursive CTE is used for recursive operations, such as navigating hierarchical data.

Why to use a Recursive CTE?

Recursive CTE is useful in following cases

• You need to generate a sequence of values based on a starting point and a recurrence relation.
• You’re working with hierarchical data structures and need to traverse or analyze them.
• You want to calculate cumulative values or totals based on a hierarchical relationship.
• You’re dealing with graph-related problems, such as finding shortest paths or identifying cycles.

Conclusion

Recursive CTEs provide a powerful way to perform recursive queries in SQL. They are ideal for hierarchical data and iterative processes, offering a simple way to handle complex recursive operations within a single query.