PostgreSQL Recursive View

Summary: in this tutorial, you will learn how to create a PostgreSQL recursive view using the CREATE RECURSIVE VIEW statement.

Introduction to the PostgreSQL recursive view

In PostgreSQL, a recursive view is a view whose defining query references the view name itself.

A recursive view can be useful in performing hierarchical or recursive queries on hierarchical data structures stored in the database.

PostgreSQL 9.3 added a new syntax for creating a recursive view specified in the standard SQL. The CREATE RECURSIVE VIEW statement is syntax sugar for a standard recursive query.

Here’s the basic syntax of the CREATE RECURSIVE VIEW statement:

CREATE RECURSIVE VIEW view_name(columns) 
AS
query;Code language: SQL (Structured Query Language) (sql)

In this syntax:

• First, specify the name of the view you want to create in the CREATE RECURSIVE VIEW clause. You can add an optional schema to the name of the view.
• Second, add a SELECT statement to define the view. The SELECT statement references the view_name to make the view recursive.

The CREATE RECURSIVE VIEW statement is equivalent to the following statement:

CREATE VIEW view_name 
AS
  WITH RECURSIVE cte_name (columns) AS (
    SELECT ...)
  SELECT columns FROM cte_name;Code language: SQL (Structured Query Language) (sql)

Creating a recursive view example

We will use the employees table created in the recursive query tutorial for the demonstration.

The following recursive query returns the employee and their managers including the CEO using a common table expression (CTE):

WITH RECURSIVE reporting_line AS (
  SELECT 
    employee_id, 
    full_name AS subordinates 
  FROM 
    employees 
  WHERE 
    manager_id IS NULL 
  UNION ALL 
  SELECT 
    e.employee_id, 
    (
      rl.subordinates || ' > ' || e.full_name
    ) AS subordinates 
  FROM 
    employees e 
    INNER JOIN reporting_line rl ON e.manager_id = rl.employee_id
) 
SELECT 
  employee_id, 
  subordinates 
FROM 
  reporting_line 
ORDER BY 
  employee_id;Code language: SQL (Structured Query Language) (sql)

Output:

 employee_id |                         subordinates
-------------+--------------------------------------------------------------
           1 | Michael North
           2 | Michael North > Megan Berry
           3 | Michael North > Sarah Berry
           4 | Michael North > Zoe Black
           5 | Michael North > Tim James
           6 | Michael North > Megan Berry > Bella Tucker
           7 | Michael North > Megan Berry > Ryan Metcalfe
           8 | Michael North > Megan Berry > Max Mills
           9 | Michael North > Megan Berry > Benjamin Glover
          10 | Michael North > Sarah Berry > Carolyn Henderson
          11 | Michael North > Sarah Berry > Nicola Kelly
          12 | Michael North > Sarah Berry > Alexandra Climo
          13 | Michael North > Sarah Berry > Dominic King
          14 | Michael North > Zoe Black > Leonard Gray
          15 | Michael North > Zoe Black > Eric Rampling
          16 | Michael North > Megan Berry > Ryan Metcalfe > Piers Paige
          17 | Michael North > Megan Berry > Ryan Metcalfe > Ryan Henderson
          18 | Michael North > Megan Berry > Max Mills > Frank Tucker
          19 | Michael North > Megan Berry > Max Mills > Nathan Ferguson
          20 | Michael North > Megan Berry > Max Mills > Kevin RamplingCode language: SQL (Structured Query Language) (sql)

You can use the CREATE RECURSIVE VIEW statement to convert a query into a recursive view as follows:

CREATE RECURSIVE VIEW reporting_line (employee_id, subordinates) AS 
SELECT 
  employee_id, 
  full_name AS subordinates 
FROM 
  employees 
WHERE 
  manager_id IS NULL 
UNION ALL 
SELECT 
  e.employee_id, 
  (
    rl.subordinates || ' > ' || e.full_name
  ) AS subordinates 
FROM 
  employees e 
  INNER JOIN reporting_line rl ON e.manager_id = rl.employee_id;Code language: SQL (Structured Query Language) (sql)

To view the reporting line of the employee id 10, you can query directly from the view:

SELECT 
  subordinates 
FROM 
  reporting_line 
WHERE 
  employee_id = 10;Code language: SQL (Structured Query Language) (sql)

Output:

                  subordinates
-------------------------------------------------
 Michael North > Sarah Berry > Carolyn HendersonCode language: SQL (Structured Query Language) (sql)

Summary

• A recursive view is a view whose defining query references the view name.
• Use the CREATE RECURSIVE VIEW statement to create a recursive view.