Update Query with Condition in SQL - sql

Question

I have a table that has 2 columns and i am trying to update another table based on these criteria:
Set the flag to 'Good' for the most duplicate keys in the Main_Key column for the same GROUP_KEY (Note we can have different Main_Keys for any GROUP_KEY)
Set the flag to 'Bad' for the least duplicate keys in the Main_Key column for the same GROUP_KEY
Set the flag to 'Don't Use' if the different Main_Keys are equal for the same GROUP_KEY
HERE IS MY TABLE
GROUP_KEY MAIN_KEY
22 4
22 4
22 55
22 55
22 55
22 55
10 10
10 10
18 87
18 22
18 22
HERE IS THE DESIRED RESULT AFTER THE UPDATE
GROUP_KEY MAIN_KEY FLAG
22 4 Bad
22 4 bad
22 55 Good
22 55 Good
22 55 Good
22 55 Good
10 10 Don't Use
10 10 Don't Use
18 87 Bad
18 22 Good
18 22 Good
I only know how to do just normal update query but not where even to start this logic. thnx for the help

Answer 1

Use:
declare #t table(GROUP_KEY int, MAIN_KEY int)
insert #t values
(22, 4),
(22, 4),
(22, 55),
(22, 55),
(22, 55),
(22, 55),
(10, 10),
(10, 10),
(18, 87),
(18, 22),
(18, 22)
select t.*, b.flag
from #t t
join
(
select a.GROUP_KEY, a.MAIN_KEY
,
case
when a.GROUP_KEY = a.MAIN_KEY
then 'Don''t Use'
when a.count = MAX(a.count) over(partition by a.GROUP_KEY)
then 'Good'
else 'Bad'
end [flag]
from
(
select t.GROUP_KEY, t.MAIN_KEY, COUNT(*) [count]
from #t t
group by t.GROUP_KEY, t.MAIN_KEY
)a
)b
on b.GROUP_KEY = t.GROUP_KEY and b.MAIN_KEY = t.MAIN_KEY
Output:
GROUP_KEY MAIN_KEY flag
----------- ----------- ---------
10 10 Don't Use
10 10 Don't Use
18 22 Good
18 22 Good
18 87 Bad
22 4 Bad
22 4 Bad
22 55 Good
22 55 Good
22 55 Good
22 55 Good
Update:
Assuming you have flag column in your table:
update #t
set flag = b.flag
from #t t
join
(
select a.GROUP_KEY, a.MAIN_KEY
,
case
when a.GROUP_KEY = a.MAIN_KEY
then 'Don''t Use'
when a.count = MAX(a.count) over(partition by a.GROUP_KEY)
then 'Good'
else 'Bad'
end [flag]
from
(
select t.GROUP_KEY, t.MAIN_KEY, COUNT(*) [count]
from #t t
group by t.GROUP_KEY, t.MAIN_KEY
)a
)b
on b.GROUP_KEY = t.GROUP_KEY and b.MAIN_KEY = t.MAIN_KEY