SQL Update with Min and Sum and Group By - sql

Question

I'm looking to write an update that will correct a flag that's gone astray.
I'd like to set the PRIMARY_FLAG = 1 for the min(TABLE_ID) grouped by ACCOUNT_NUMBER where the sum(PRIMARY_FLAG) <> 1 grouped by ACCOUNT_NUMBER.
Here is what the table looks like now:
TABLE_ID ACCOUNT_NUMBER PRIMARY_FLAG
-------- -------------- ------------
1 ABC123 0
2 ABC123 1
3 ABC123 0
4 987XYZ 0
5 987XYZ 0
6 987XYZ 0
7 5A5B5C 1
8 5A5B5C 1
9 5A5B5C 0
10 5A5B5C 0
Here's what I want it to look like after the update:
TABLE_ID ACCOUNT_NUMBER PRIMARY_FLAG
-------- -------------- ------------
1 ABC123 0
2 ABC123 1
3 ABC123 0
4 987XYZ 1
5 987XYZ 0
6 987XYZ 0
7 5A5B5C 1
8 5A5B5C 0
9 5A5B5C 0
10 5A5B5C 0
Scenario 1 - TABLE_ID 1, 2, 3 with ACCOUNT_NUMBER = ABC123 is already correct and I do not want the update to touch it.
Scenario 2 - ACCOUNT_NUMBER = 987XYZ, none of the TABLE_ID have a PRIMARY_FLAG = 1, so update will set PRIMARY_FLAG = 1 where TABLE_ID = 4
Scenario 3 - ACCOUNT_NUMBER = 5A5B5C has multiple TABLE_ID = 1, so update would leave TABLE_ID 7 but SET PRIMARY_FLAG = 0 where TABLE_ID = 8
Help is appreciate!

Answer 1

Here is a generic method:
update t
set primary_flag = (case when t.id = (select min(t2.id)
from t t2
where t2.account_number = t.account_number
)
then 1 else 0
end)
where t.account_number in (select t2.acount_number
from t t2
group by t2.acount_number
having sum(t2.primary_flag) <> 1
);
This is standard SQL and will work in most databases. However, specific databases might have more concise or efficient methods of doing the same thing.

Answer 2

Join the table to the query that returns the minimum table_id for each account_number that needs to be updated:
update t
set t.primary_flag = case when t.table_id = g.minid then 1 else 0 end
from tablename t inner join (
select account_number, min(table_id) minid
from tablename
group by account_number
having sum(primary_flag) <> 1
) g on g.account_number = t.account_number;
See the demo.
Or with a CTE:
with cte as (
select *,
row_number() over (partition by account_number order by table_id) rn,
sum(primary_flag) over (partition by account_number) total
from tablename
)
update cte
set primary_flag = case when rn = 1 then 1 else 0 end
where total <> 1
See the demo.
To avoid unnecessary updates for the rows that do not need to be updated:
with cte as (
select *,
row_number() over (partition by account_number order by table_id) rn,
sum(primary_flag) over (partition by account_number) total
from tablename
)
update cte
set primary_flag = abs(primary_flag - 1)
where (total <> 1) and ((rn = 1 and primary_flag = 0) or (rn > 1 and primary_flag = 1))
See the demo.
Results:
> TABLE_ID | ACCOUNT_NUMBER | PRIMARY_FLAG
> -------: | :------------- | -----------:
> 1 | ABC123 | 0
> 2 | ABC123 | 1
> 3 | ABC123 | 0
> 4 | 987XYZ | 1
> 5 | 987XYZ | 0
> 6 | 987XYZ | 0
> 7 | 5A5B5C | 1
> 8 | 5A5B5C | 0
> 9 | 5A5B5C | 0
> 10 | 5A5B5C | 0