Stephen Tableau - 6 months ago 37

SQL Question

I'm having trouble formulating a query for the following problem:

For pair values that have a certain score, how do you group them in way that will only return distinct pair values with the best respective scores?

For example, lets say I have a table with the following row values:

`(t1,p1,65)`

(t1,p2,60)

(t1,p3,20)

(t2,p1,60)

(t2,p2,59)

(t2,p3,15)

The first two columns indicate the pair values and the third column represents the pair score.The best score is

`(t1,p1,65)`

The next best score is

`(t2,p2,59)`

`(t1,p2)`

`t2,p1)`

This results in the distinct pair score values of:

`(t1,p1,65)`

(t2,p2,59)

Is there any way to generate this result with just a query? I've tried to think of ways of grouping and partitioning the results, but since there has to be some accounting of values already used according to score rank, I'm finding this very difficult to approach.

To generate the data:

`with t(t, p, score) as (`

(values ('t1','p1',65),

('t1','p2',60),

('t1','p3',20),

('t2','p1',60),

('t2','p2',59),

('t2','p3',15)

))

select t.* from t;

Answer

It is relatively simple using the stored function:

```
--drop function if exists f();
--drop table if exists t;
create table t(x text,y text, z int);
insert into t values
('t1','p1',65),
('t1','p2',60),
('t1','p3',20),
('t2','p1',60),
('t2','p2',59),
('t2','p3',15)/*,
('t3','p1',20),
('t3','p2',60),
('t3','p3',40)*/;
create function f() returns setof t immutable language plpgsql as $$
declare
ax text[];
ay text[];
r t;
begin
ax := '{}'; ay := '{}';
loop
select * into r
from t
where x <> all(ax) and y <> all(ay)
order by z desc, x, y limit 1;
exit when not found;
ax := ax || r.x; ay := ay || r.y;
return next r;
end loop;
end $$;
select * from f();
╔════╤════╤════╗
║ x │ y │ z ║
╠════╪════╪════╣
║ t1 │ p1 │ 65 ║
║ t2 │ p2 │ 59 ║
╚════╧════╧════╝
```

However if uncomment the third bunch of the values the result will be different:

```
╔════╤════╤════╗
║ x │ y │ z ║
╠════╪════╪════╣
║ t1 │ p1 │ 65 ║
║ t3 │ p2 │ 60 ║
║ t2 │ p3 │ 15 ║
╚════╧════╧════╝
```

Upd: and the equivalent using recursive CTE on the same test data:

```
with recursive r as (
(select x, y, z, array[x] as ax, array[y] as ay from t order by z desc, x, y limit 1)
union all
(select t.x, t.y, t.z, r.ax || t.x, r.ay || t.y from t, r
where not (t.x = any(r.ax) or t.y = any(r.ay))
order by t.z desc, t.x, t.y limit 1))
select * from r;
```