SQL 过滤独特的结果

Question

I am trying to get an SQL statement that will output a unique part number eg no duplicates However I want the type as Purchased is the default and when there isnt a Purchased part it defults back to Manufactured. NOTE all parts can be purchased

The result I require is to only show unique part numbers eg 1 to 10 and Purchased is the default Type

Table1

Part_number | Type         | Cost
Part 1 |    Manufactured   | £1.00
Part 1 |    Purchased      | £0.56
Part 2 |    Manufactured   | £1.26
Part 2 |    Purchased      | £0.94
Part 3 |    Manufactured   | £0.36
Part 3 |    Purchased      | £0.16
Part 4 |    Manufactured   | £1.00 
Part 4 |    Purchased      | £1.50
Part 5 |    Manufactured   | £1.65
Part 6 |    Manufactured   | £1.98 
Part 7 |    Manufactured   | £0.15
Part 8 |    Manufactured   | £0.45
Part 9 |    Manufactured   | £1.20
Part 9 |    Purchased      | £0.80
Part 10|    Manufactured   | £1.00

This is the result I am hoping to get back

Part_number | Type         | Cost
Part 1 |    Purchased      | £0.56
Part 2 |    Purchased      | £0.94
Part 3 |    Purchased      | £0.16
Part 4 |    Purchased      | £1.50
Part 5 |    Manufactured   | £1.65
Part 6 |    Manufactured   | £1.98 
Part 7 |    Manufactured   | £0.15
Part 8 |    Manufactured   | £0.45
Part 9 |    Purchased      | £0.80
Part 10|    Manufactured   | £1.00

I have tried loads of different techniques but am not getting the result.

I am guessing that I will need to create temp tables that are filtered and then join the tables together but I really don't know.

Any help will be apricated

Answer 1

One method uses not exists . Assuming you have at most two rows per part:

select t.*
from t
where t.type = 'Purchased' or
      (t.type = 'Manufactured' and
       not (exists (select 1 from t t2 where t2.part_number = t.part_number and t2.type = 'Purchased')
           )
      );

There are other fun ways to handle this. For instance, an aggregation approach:

select part_number,
       max(type) as type,
       coalesce(max(case when type = 'Purchased' then cost end),,
                max(cost)
               ) as cost
from t
group by part_number;

Answer 2

You could also just grab the first row in each group by sorting them. This would make it easier when there are other columns of data to bring back.

with data as (
    select *, row_number() over (
        partition by part_number
        order by case when t.type = Purchased then 1 else 2 end) as rn
    from t
)
select * from data where rn = 1;

If there are other types this would work as well although you would want to tweak it if there are more than two per part.