Check if values are the same for each row over critieras in different other columns

DB-Fiddle

CREATE TABLE campaigns (
    id SERIAL PRIMARY KEY,
    campaign VARCHAR,
    supplier VARCHAR,
    plan_quantity DECIMAL
);

INSERT INTO campaigns
(campaign, supplier, plan_quantity)
VALUES 
('C001', 'supplier_a', '500'),
('C001', 'supplier_a', '500'),
('C001', 'supplier_b', '500'),

('C002', 'supplier_a', '600'),
('C002', 'supplier_b', '700'),

('C003', 'supplier_c', '100'),
('C003', 'supplier_c', '100'),

('C004', 'supplier_a', '900'),
('C004', 'supplier_c', '800'),
('C004', 'supplier_d', '250'),
('C004', 'supplier_d', '250'),

('C005', 'supplier_b', '380'),
('C005', 'supplier_b', '270'),
('C005', 'supplier_d', '590');

Expected Result:

campaign  |   supplier   |   plan_quantity   |      check         |
----------|--------------|-------------------|--------------------|-------
  C001    |  supplier_a  |        500        |       same         |
  C001    |  supplier_a  |        500        |       same         |
  C001    |  supplier_b  |        500        |    non-relevant    |
----------|--------------|-------------------|--------------------|-------
  C002    |  supplier_a  |        600        |    non-relevant    |
  C002    |  supplier_b  |        700        |    non-relevant    | 
----------|--------------|-------------------|--------------------|-------
  C003    |  supplier_c  |        100        |       same         |
  C003    |  supplier_c  |        100        |       same         |
----------|--------------|-------------------|--------------------|-------
  C004    |  supplier_a  |        900        |    non-relevant    |
  C004    |  supplier_c  |        800        |    non-relevant    |
  C004    |  supplier_d  |        250        |       same         |
  C004    |  supplier_d  |        250        |       same         |
----------|--------------|-------------------|--------------------|-------
  C005    |  supplier_b  |        380        |     different      |
  C005    |  supplier_b  |        270        |     different      |
  C005    |  supplier_d  |        590        |    non-relevant    |

In case a supplier appears mutliple times per campaign I want to see in colum check if the plan_quantity for this supplier is the same in every row.

I am getting close to the result when I modify the query from this question:

SELECT
campaign AS campaign,
supplier AS supplier,
plan_quantity AS plan_quantity,
(CASE WHEN 
MIN(plan_quantity) OVER(PARTITION BY supplier, campaign) = MAX(plan_quantity) OVER(PARTITION BY supplier, campaign) 
THEN 'same' ELSE 'different' END) AS check
FROM campaigns
ORDER BY 1,2,3;

However, I have no clue how I can add the description non-relevant to the query in case a supplier does not appear mutliple times per campaign.

Do you have any idea?

>Solution :

You can slightly modify the query with an additional case expression for when the check is the same and indicate any with a count of 1 are non-relevant:

select 
  campaign,
  supplier,
  plan_quantity, 
  case when 
      Min(plan_quantity) over(partition by campaign, supplier) 
        = Max(plan_quantity) over(partition by campaign, supplier) 
    then 
        case when Count(*) over(partition by campaign, supplier) = 1 
         then 'non-relevant' else 'same' 
        end
    else 'different'
    end as Check
from campaigns
order by 1,2,3;