August 3, 2023
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
PostgreSQL 9.3 added a new syntax for creating recursive views specified in the standard SQL. The CREATE RECURSIVE VIEW statement is syntax sugar for a standard recursive query.
The following illustrates the CREATE RECURSIVE VIEW syntax:
CREATE RECURSIVE VIEW view_name(columns) AS
SELECT columns;
First, specify the name of the view that you want to create in the CREATE RECURSIVE VIEW clause. You can add an optional schema-qualified to the name of the view.
Second, add the SELECT statement to query data from base tables. The SELECT statement references the view_name to make the view recursive.
This statement above is equivalent to the following statement:
CREATE VIEW view_name
AS
WITH RECURSIVE cte_name (columns) AS (
SELECT ...)
SELECT columns FROM cte_name;
Creating 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 up to the CEO level using a common table expression or 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;
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 Rampling
(20 rows)
You can use the CREATE RECURSIVE VIEW statement to convert the 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;
To see the reporting line of the employee id 10, you query directly from the view:
SELECT
subordinates
FROM
reporting_line
WHERE
employee_id = 10;
subordinates
-------------------------------------------------
Michael North > Sarah Berry > Carolyn Henderson
(1 row)
In this tutorial, you have learned how to create PostgreSQL recursive view based on a recursive query.