sql - Simple SQL Query framing

Question

I'm relatively new to SQL and postgres. I've relation R1 and R2 with few columns , one each being some measurement,say R1.m and R2.m . Now i need to find for every row in R1 , the minimum of some function f of (R1.m and R2.m) , f(R1.m,R2.m) so that the code is something like select R1.name, R2.name from R1,R2 where f(R1.m,R2.m) < ....... . I'm not able to fill up this query . I need a row of R2 for every row in R1 which minimizes the function. I know this is simple , just cant work it out for a newbie . (In postgres)

Answer 1

Not sure I undrestand your question correctly, try something like this:

with 

r1 as (select generate_series(1,10,1) as m)
,r2 as (select generate_series(10,20,1) as m)

,cte as (
   select
      r1.m as "r1.m",
      r2.m as "r2.m",
      sin(r1.m * r2.m) f_r1r2,
      dense_rank() over (partition by r1.m order by sin(r1.m * r2.m)) as f_r1r2_rank
   from
      r1,
      r2
   )

select
   *
from
   cte
where
   f_r1r2_rank = 1

This will:

1. Create cross product or r1 and r2 (get all possible combinations of r1.m and r2.m)
2. Calculate function f (in my case I used sin(r1.m*r2.m) as an example)
3. Calculate rank for every set of combination of r1 and r2
4. Select only those where rank = 1 (minimum)

In other words this will answer question: "What r2.m should I use to get minimum of function f(r1.m,r2.m) given r1.m?".

Because you have not provided sample data, I've generated my own. To make it work, you have to use something like:

with     
cte as (
   select
      r1.m as "r1.m",
      r2.m as "r2.m",
      sin(r1.m * r2.m) f_r1r2,
      dense_rank() over (partition by r1.m order by sin(r1.m * r2.m)) as f_r1r2_rank
   from
      r1,
      r2
   )

select
   *
from
   cte
where
   f_r1r2_rank = 1

and of course, you have to replace sin(r1.m * r2.m) with your own function.

Answer 2

If I understand it

select distinct on (r1.name)
    r1.name, r2.name, f(r1.m, r2.m)
from r1 cross join r2
order by r1.name, f(r1.m, r2.m)

distinct on will return one only r1.name, the first one computed from the order by clause.