Recursive Queries in SQL refer to queries that repeatedly execute until they reach a certain condition. They are useful for hierarchical data, such as a company’s organizational structure or a family tree. Recursive queries use a WITH RECURSIVE clause in SQL.
The basic syntax for recursive queries is as follows:
WITH RECURSIVE cte_name (column1, column2, ...) AS ( -- non-recursive term SELECT column1, column2, ... FROM table_name WHERE condition UNION ALL -- recursive term SELECT column1, column2, ... FROM cte_name JOIN table_name ON ... WHERE condition ) SELECT column1, column2, ... FROM cte_name;
cte_name is the name of the Common Table Expression (CTE), and
column2, etc. are the columns you want to select. The
condition parameters are the same as in regular SQL queries.
The WITH RECURSIVE clause has two parts: the non-recursive term and the recursive term. The non-recursive term is executed once at the beginning of the query, while the recursive term is executed repeatedly until it reaches a certain condition.
Let’s look at an example to better understand how recursive queries work. Suppose we have the following table of employees in a company:
CREATE TABLE employees ( id INTEGER PRIMARY KEY, name TEXT NOT NULL, manager_id INTEGER REFERENCES employees(id) ); INSERT INTO employees (id, name, manager_id) VALUES (1, 'Alice', NULL), (2, 'Bob', 1), (3, 'Charlie', 2), (4, 'Dave', 3), (5, 'Eve', 2), (6, 'Frank', 1), (7, 'Grace', 6), (8, 'Heidi', 7), (9, 'Ivan', 7);
We can use a recursive query to generate a report of all the employees in the company and their managers. The non-recursive term selects the top-level employees (those without a manager), while the recursive term selects the employees that report to each top-level employee.
WITH RECURSIVE employee_hierarchy (id, name, level) AS ( -- non-recursive term SELECT id, name, 0 FROM employees WHERE manager_id IS NULL UNION ALL -- recursive term SELECT e.id, e.name, eh.level + 1 FROM employee_hierarchy eh JOIN employees e ON e.manager_id = eh.id ) SELECT id, name, level FROM employee_hierarchy;
The output of this query would be:
id name level -------------------- 1 Alice 0 2 Bob 1 6 Frank 1 3 Charlie 2 5 Eve 2 7 Grace 2 4 Dave 3 8 Heidi 3 9 Ivan 3
As you can see, the query has generated a report of all the employees in the company, along with their level in the organizational structure.
Recursive queries can be very powerful for handling hierarchical data, but they can also be computationally expensive and difficult to understand. It’s important to use them judiciously and to make sure that they are optimized for performance.