Add Default Rows in Postgresql - postgresql

Question

I want to insert default rows into a result set if the LEFT JOIN is NULL.
For example if Jane has no roles, I want to return some default ones in the results.
A query like this will return the following:
SELECT * FROM employees LEFT OUTER JOIN roles ON roles.employee_id = employees.id
Employee ID | Employee Name | Role ID | Role Name
1 | John | 1 | Admin
1 | John | 2 | Standard
2 | Jane | NULL | NULL
I want to return:
Employee ID | Employee Name | Role ID | Role Name
1 | John | 1 | Admin
1 | John | 2 | Standard
2 | Jane | NULL | Admin
2 | Jane | NULL | Standard
Is there a good way to do this in PostgreSQL?

Answer 1

I think you're looking for
SELECT e.*, r.*
FROM employees e
JOIN roles r ON r.employee_id = e.id
UNION ALL
SELECT e.*, NULL, default_name
FROM employees e
JOIN (VALUES ('Admin'), ('Standard')) AS roles(default_name)
WHERE NOT EXISTS (
SELECT *
FROM roles r
WHERE r.employee_id = e.id
)
I don't think there's a (good) way around the UNION because a LEFT JOIN introduces only a single row per unmatched row. You might be able to lift out the join against the employees table though:
SELECT e.*, r.*
FROM employees e,
LATERAL (
SELECT r.id, r.name
FROM roles r
WHERE r.employee_id = e.id
UNION ALL
SELECT NULL, default_name
FROM (VALUES ('Admin'), ('Standard')) AS roles(default_name)
WHERE NOT EXISTS (
SELECT *
FROM roles r
WHERE r.employee_id = e.id
)
)