Compare values of timestamps and assign a value to each of them in case they have changed

Question

DB-Fiddle

CREATE TABLE operations (
    id int auto_increment primary key,
    time_stamp DATE,
    product VARCHAR(255),
    plan_week VARCHAR(2         


        

Answer 1

I would just use window functions:

select o.*,
       (case when min(plan_week) over (partition by product) = max(plan_week) over (partition by product)
             then 'no' else 'yes'
        end) as switched
from operations o;

Answer 2

If I understand your requirement correctly, your week_switch result row depends on just your product, not its timestamp.

So compute it in a subquery.

  SELECT product,
         (CASE 
          WHEN MIN(plan_week) <> MAX(plan_week) THEN 'yes' 
          ELSE 'no' END)  AS week_switch
    FROM operations
   GROUP BY product

Then JOIN that subquery to your details (fiddle).

WITH switched AS (
  SELECT product,
         CASE
         WHEN MIN(plan_week) <> MAX(plan_week) THEN 'yes'
         ELSE 'no' END AS week_switch
    FROM operations
   GROUP BY product
)
SELECT
operations.time_stamp,
operations.product,
operations.plan_week,
switched.week_switch
FROM operations
LEFT JOIN switched ON operations.product = switched.product
WHERE time_stamp in ('2020-01-01', '2020-03-15')
GROUP BY 1,2;