Sql with recursive example

Let’s take some examples of using recursive CTEs. A) Simple SQL Server recursive CTE example. We have to create views, cursors and so on, but using a CTE in SQL Server is a better solution to retrieve hierarchy-based data and in this tip, I will show you how. Common Table Expressions (CTE) have two types, recursive and non- recursive.

We will see how the recursive CTE works with examples in this tip.

Recursive joins are often used to obtain parent-child data. You must combine the recursive member with the anchor member using the union all set operator. This site shows an example of the 11g Rnew with clause syntax.

I never use CTE with recursion. I was just reading an article on it. How to select using WITH RECURSIVE clause.

A common table expression that includes references to itself (a recursive common table expression) is not supported. Specifying more than one WITH clause in a CTE is not allowed.

For example , if a CTE query definition contains a subquery, that subquery cannot contain a nested WITH clause that defines another CTE. By using recursion , we’re able to use the self-referring nature of the CTE to continue to dig into deeper levels of the BOM. Much like the counting example we showed previously, with the following recursive CTE, the common table expression is repeatedly called until the termination condition is met. They are special cases of more general recursive fixpoint queries, which compute transitive closures. The SQL standard defines a special syntax for common table expressions to enable recursive processing.

But, if you have a business need to walk or explode hierarchies in your database, recursive SQL will likely be your most efficient option. Consider a table of parts with associated subparts and the quantity of subparts required by each part. For more information about recursive SQL , refer to Creating recursive SQL by using common table expressions. One fun thing about recursive WITH, aka recursive subquery refactoring, is the ease with which we can implement a recursive algorithm in SQL. Let’s warm up with a classic example of recursion : finding the factorial of a number.

In the example above, the working table has just a single row in each step, and it takes on the values from through 1in successive steps. In the 100th step, there is no output because of the WHERE clause, and so the query terminates. WITH common_table_expression (Transact-SQL ). The following example uses multiple anchor and recursive members to return all the ancestors of a specified person.

But the recursive element of this processing can be heavy for larger tables with deep hierarchies and should thus be used carefully. But in smaller sets, or when later processing is difficult, you’ve got a key tool in the CTE’s ability to call itself. In the examples we will use PostgreSQL, which requires the RECURSIVE keyword in recursive CTE definitions but it is optional for other databases.

Here is some sample data in CSV format for the remaining examples. A recursive query part is a query that references to the CTE name, therefore, it is called a recursive member. A termination condition that ensures the recursion stops when the recursive member returns no row. The recursive member is joined with the anchor member by aUNION ALL or UNION DISTINCT operator.

In this example also, we are using the SQL CTE recursive. However, this time we are going to modify the above example to show the hierarchical level visually. In the following examples , you will learn how to harness the power of a recursive CTE query by fulfilling a common business requirement, retrieving hierarchical data. By the time the final query is complete you will be able to easily determine how many levels from the top executive each employee is.

In my previous posts I’ve discussed this topic with some good examples , link. An incorrectly composed recursive CTE may cause an infinite loop. So recursive CTEs should be designed very carefully and the recursion level should be checked.

To prevent it to run infinitely SQL Server’s default recursion level is set to 100. To keep it simple, the following example only references the aggregations once, where the SQL WITH clause is normally used when an aggregation is referenced multiple times in a query. Sql server CTE and recursion.

Here is an example of a request to see the names of all stores with above-average sales.