SQL query from Two tables and a link table to return all of table c for each row of table a with nulls where the is no link in table b

Question

Given the schema:

Table a: worker_role_desription,role
Linking Table b: role,stepid, permission
Table c: stepid, step_description

I need to return a full list of step_descriptions from c for each role from a with the permission from b if it exists and 'none' if it doesn't.

I have tried various ways of joining the tables but have failed to return what I need.

Answer 1

You want something like:

SELECT worker_role,
       step_description,
       COALESCE(permission, 'none') AS permission
FROM   a
       CROSS JOIN c
       LEFT OUTER JOIN b
       ON (a.role = b.role AND c.stepid = b.stepid)
ORDER BY
       a.role,
       c.stepid

Which, for the sample data:

CREATE TABLE a (worker_role, description, role PRIMARY KEY) AS
SELECT 'Role'|| LEVEL, 'Role Descr'||LEVEL, LEVEL FROM DUAL CONNECT BY LEVEL <= 3;

CREATE TABLE c (stepid PRIMARY KEY, step_description) AS
SELECT LEVEL, 'Step Descr'||LEVEL FROM DUAL CONNECT BY LEVEL <= 3;

CREATE TABLE b(role, stepid, permission) AS
SELECT 1, 1, 'R' FROM DUAL UNION ALL
SELECT 1, 2, 'R' FROM DUAL UNION ALL
SELECT 1, 3, 'RW' FROM DUAL UNION ALL
SELECT 2, 1, 'R' FROM DUAL UNION ALL
SELECT 2, 3, 'W' FROM DUAL;

ALTER TABLE b
  ADD CONSTRAINT b__role__fk FOREIGN KEY (role) REFERENCES a(role)
  ADD CONSTRAINT b__stepid__fk FOREIGN KEY (stepid) REFERENCES c(stepid);

Outputs:

WORKER_ROLE	STEP_DESCRIPTION	PERMISSION
Role1	Step Descr1	R
Role1	Step Descr2	R
Role1	Step Descr3	RW
Role2	Step Descr1	R
Role2	Step Descr2	none
Role2	Step Descr3	W
Role3	Step Descr1	none
Role3	Step Descr2	none
Role3	Step Descr3	none

db<>fiddle here