Select rows based on multiple conditions - sql

Question

I have a table from which I have to select some rows based on the following conditions.
If more than one row exists with same DocumentRef, then select all the rows if BlockNumber is empty for all rows
If more than one row exists with same DocumentRef, then select only 1 row (ordered by DocumentId asc) with BlockNumber IS NOT EMPTY
If only one row exists with DocumentRef, select it irrespective of anything
Table:
I was trying to group it by DocumentRef and filter with having but having can only have aggregate functions. I think I will have to provide multiple conditions in having separated by OR. Please give me some direction.

Answer 1

Use window functions:
select t.*
from (select t.*,
sum(case when blocknumber is not null then 1 else 0 end) over (partition by documentref) as num_bn_notnull,
rank() over (partition by documentref
order by (case when blocknumber is not null then documentid end) desc nulls last
) as rnk
from t
) t
where num_bn_notnull = 0 or
rnk = 1;
Or, you can use exists clauses:
select t.*
from t
where not exists (select 1
from t t2
where t2.documentref = t.documentref and
t2.blocknumber is not null
) or
t.documentid = (select max(t2.documentid)
from t t2
where t2.documentref = t.documentref and
t2.blocknumber is not null
);
This can take advantage of an index on (documentref, blocknumber, documentid).
Actually, by a quirk of the SQL language, I think this works as well:
select t.*
from t
where t.documentid >= any (select t2.documentid
from t t2
where t2.documentref = t.documentref and
t2.blocknumber is not null
order by t2.documentid
fetch first 1 row only
);
The subquery returns an empty set if all blocknumbers are NULL. By definition, any document id matches the condition on an empty set.

Answer 2

Join the table to a query that returns for each documentref the maximum documentid for all the blocknumbers that are not null or null if they are all null:
select t.*
from tablename t inner join (
select
documentref,
max(case when blocknumber is not null then documentid end) maxid
from tablename
group by documentref
) d on d.documentref = t.documentref
and t.documentid = coalesce(d.maxid, t.documentid)
See the demo.
Results:
> DOCUMENTID | DOCUMENTREF | WARDID | BLOCKNUMBER
> ---------: | ----------: | -----: | ----------:
> 203962537 | 100000126 | B | A
> 203962538 | 100000130 | B | A
> 203962542 | 100000151 | null | null
> 203962543 | 100000151 | null | null
> 203962544 | 100000180 | B | A
> 203962546 | 100000181 | B | A
> 203962551 | 100000185 | null | null
> 203962552 | 100000186 | B | A