mysql - Grouped query for find only one record (employee_id = 1 or null)

Question

My db info is

I wont to get for each weekday unique record if employee_id has value that record, else employee_id is Null. I try this query

SELECT 
  * 
FROM
  `calendar` 
WHERE `employee_id` = 1 
  OR `employee_id` IS NULL 
GROUP BY `weekday` 

result is

But I expect

Can you help me? Thanks

Answer 1

Try this out:

SELECT 
  b.calendar_id,
  a.employee_id,
  a.weekday 
FROM
  (SELECT 
    `weekday`,
    MAX(employee_id) AS employee_id 
  FROM
    calendar 
  WHERE employee_id = 1 
    OR employee_id IS NULL 
  GROUP BY `weekday`) a 
  LEFT JOIN calendar b 
    ON a.weekday = b.weekday 
    AND a.employee_id = b.employee_id ;

Let me know in case of any clarifications.