How to find most frequent code(varchar) from a table - sql

Question

I would like to find most frequent code within CodeID which is in same code_group from a table.
For example, from original table
ID CodeID Name Code Code_group
1 1 A 101 0
2 1 A 102 0
3 1 B 102 0
4 2 C 201 0
5 2 C 201 0
6 2 D 202 0
7 2 E 202 0
8 3 F 101 1
9 3 G 103 1
10 3 G 104 1
11 3 G 104 1
I want output like the below.
ID CodeID Name Code Code_group Selected_code
1 1 A 101 0 102
2 1 A 102 0 102
3 1 B 102 0 102
4 2 C 201 0 NULL
5 2 C 201 0 NULL
6 2 D 202 0 NULL
7 2 E 202 0 NULL
8 3 F 101 1 104
9 3 G 103 1 104
10 3 H 104 1 104
11 3 H 104 1 104
Even though code of 8th ID is same in CodeID: 1,it is not in the same Code_group.
So For CodeID: 1, Selected_code would be 102.
it must be counted within exactly same Code_group.
=======================================
I have tried it like the below. I should not use ID for this one.
From TableA
with m as
(
select
CodeID,
Name,
Code,
Code_group,
cnt,
Selected_code = ROW_NUMBER() over (partition by Code_group order by cnt desc)
from( select CodeID, Name, Code,Code_group
,count(*) over (partition by Code,CodeID) as cnt from tableA
group by CodeID, Name, Code, Code_group,
) as t
group by CodeID,
Name,
Code,
Code_group, cnt
)
select a.CodeID,
a.Name,
a.Code,
a.Code_group, b.Code as Selected_code, cnt
from(select
CodeID,
Name,
Code,
Code_group,Selected_code,
cnt
from m) as a left outer join
(select CodeID,
Name,
Code,
Code_group,Selected_code,
cnt
from m where selected_Code=1) as b on a.CodeID = b.CodeID and a.Code_Group = b.Code_Group
order by a.CodeID, a.Code_Group
The problem of this is
With statment makes my table distinct. It shows only one row if there is exactly same data such as ID 1,2.
Also, I cannot make NULL if there is exactly same frequencies.
What should I add to get my desired output?
Or is there any better approach for this?

Answer 1

CTE cte find the highest frequency code by Code_group and CodeID using dense_rank()
CTE selected check for any Code with same frequency and exclude them.
Final query just select from the original table and LEFT JOIN the selected
with
cte as
(
select Code_group, CodeID, Code
from
(
select Code_group, CodeID, Code,
r = dense_rank() over (partition by Code_Group, CodeID
order by count(*) desc)
from tableA
group by Code_group, CodeID, Code
) c
where c.r = 1
),
selected as
(
select Code_group, CodeID, Code
from
(
select Code_group, CodeID, Code,
cnt = count(*) over (partition by Code_group, CodeID)
from cte
) s
where s.cnt = 1
)
select a.*,
Selected_Code = s.Code
from tableA a
left join selected s on a.Code_Group = s.Code_Group
and a.CodeID = s.CodeID;
db<>fiddle demo