Select IDs from table where col_2 is not null (duplicate id) - mysql

Question

I have a table that contains the following data
ID | Col_2
A | 'ABC'
A | 'GHI'
A | null
B | 'null'
B | 'HJH'
B | 'NBN'
C | null
I have two cases to cater :
Duplicate Ids:
Incase of duplicate ids I only want those IDs which do not have null in col_2
E.g.
Query should return :
A | 'ABC'
A | 'GHI'
B | 'HJH'
B | 'NBN'
Non Duplicate Id:
Incase of non duplicate id the query should return result irrespective of the value present in col_2
So the final result of the query should be
ID | Col_2
A | 'ABC'
A | 'GHI'
B | 'HJH'
B | 'NBN'
C | null
I have managed to create the following query where it is fulfilling the duplicate id case not the non duplicate case.
Query :
select id,col_2
from mytable
group by id,col_2
having (sum(case when col_2 is not null then 1 else 0 end) > 0)
What changes should be made in the query to cater the non duplicate case also.
Thanks in advance!!!

Answer 1

Assuming NULL is NULL and not a string and that you have only one NULL value per id, you can do something like this:
select t.*
from t
where t.col_2 is not null or
not exists (select 1 from t t2 where t2.id = t.id and t2.col_2 is not null);
If your null values can be duplicated and you want only one row for them, then tweak this to:
select t.*
from t
where t.col_2 is not null
union all
select distinct t.*
from t
where not exists (select 1 from t t2 where t2.id = t.id and t2.col_2 is not null);
Here is a db<>fiddle.
For performance, you want an index on (id, col_2).
If you just want the col_2 values for each id, you can concatenate them on each row:
select id, group_concat(col_2)
from t
group by id;
Another alternative uses window functions:
select t.id, col_2
from (select t.*,
rank() over (partition by id order by col_2 is not null desc) as seqnum
from t
) t
where seqnum = 1;