mysql select distinct if condition exists - mysql

Question

I have this table :
std_id std_type std_dept target
1 type-1 ALL 15
2 type-1 HRD 10
3 type-2 ALL 1
4 type-2 ACCTG 5
5 type-3 ALL 5
6 type-4 ALL 25
std_dept with value ALL means std_target for each std_type are valid for all dept
std_dept with specific dept value, override the value form point (1).
So, my problem is lets say I am from HRD dept and I want to have the result set as follow:
std_id std_type std_dept target
2 type-1 HRD 10
3 type-2 ALL 1
5 type-3 ALL 5
6 type-4 ALL 25
I really like to show my work, but I've got no clue at all, I don't know how to distinctly select * from std_table (for each std_type) where std_dept='HRD' if exist else get from std_dept='ALL'.
I need pointers if not the code

Answer 1

You could use NOT EXISTS and correlated subquery:
SELECT *
FROM tab
WHERE std_dept = 'HRD'
UNION ALL
SELECT *
FROM tab t1
WHERE std_dept = 'ALL'
AND NOT EXISTS (SELECT *
FROM tab t2
WHERE t2.std_dept = 'HRD'
AND t1.std_type = t2.std_type);
DBFiddle Demo

Answer 2

It sounds like you want to weight it, so if you're from HRD then you want HRD or ALL, and if you want ACCTG then you want ACCTG or ALL, for types 1-4.
You could group by std_type to make them distinct, and order by that to make the order correct. For the weighting thing, you could create a column which is the result of a comparison against the same value you're using in your WHERE clause, e.g. something like:
SELECT std_id, DISTINCT(std_type) AS std_type,
std_dept, target,
IF (std_dept = 'HRD', 50, 10) AS weight
FROM mytable
WHERE std_dept IN ('ALL', 'HRD')
GROUP BY std_id, std_dept, target, weight
ORDER BY std_type