Selecting distinct values for multiple columns

Question

I have a table where many pieces of data match to one in another column, similar to a tree, and then data at the 'leaf' about each specific leaf

eg

Food Group      Name       Caloric Value  
Vegetables      Broccoli   100  
Vegetables      Carrots    80    
Fruits          Apples     120  
Fruits          Bananas    120  
Fruits          Oranges    90

I would like to design a query that will return only the distinct values of each column, and then nulls to cover the overflow

eg

Food group    Name     Caloric Value  
Vegetables    Broccoli 100  
Fruit         Carrots  80  
              Apples   120  
              Bananas  90  
              Oranges   

I'm not sure if this is possible, right now I've been trying to do it with cases, however I was hoping there would be a simpler way

Answer 1

Seems like you are simply trying to have all the distinct values at hand. Why? For displaying purposes? It's the application's job, not the server's. You could simply have three queries like this:

SELECT DISTINCT [Food Group] FROM atable;

SELECT DISTINCT Name FROM atable;

SELECT DISTINCT [Caloric Value] FROM atable;

and display their results accordingly.

But if you insist on having them all in one table, you might try this:

WITH atable ([Food Group], Name, [Caloric Value]) AS (
  SELECT 'Vegetables', 'Broccoli', 100  UNION ALL
  SELECT 'Vegetables', 'Carrots',  80   UNION ALL
  SELECT 'Fruits',     'Apples',   120  UNION ALL
  SELECT 'Fruits',     'Bananas',  120  UNION ALL
  SELECT 'Fruits',     'Oranges',  90   
),
atable_numbered AS (
  SELECT
    [Food Group], Name, [Caloric Value],
    fg_rank = DENSE_RANK() OVER (ORDER BY [Food Group]),
    n_rank  = DENSE_RANK() OVER (ORDER BY Name),
    cv_rank = DENSE_RANK() OVER (ORDER BY [Caloric Value])
  FROM atable
)
SELECT
  fg.[Food Group],
  n.Name,
  cv.[Caloric Value]
FROM (
  SELECT fg_rank FROM atable_numbered  UNION
  SELECT n_rank  FROM atable_numbered  UNION
  SELECT cv_rank FROM atable_numbered
) r (rank)
  LEFT JOIN (
    SELECT DISTINCT [Food Group], fg_rank
    FROM atable_numbered) fg ON r.rank = fg.fg_rank
  LEFT JOIN (
    SELECT DISTINCT Name, n_rank
    FROM atable_numbered) n  ON r.rank = n.n_rank
  LEFT JOIN (
    SELECT DISTINCT [Caloric Value], cv_rank
    FROM atable_numbered) cv ON r.rank = cv.cv_rank
ORDER BY r.rank

Answer 2

I guess what I would want to know is why you need this in one result set? What does the code look like that would consume this result? The attributes on each row have nothing to do with each other. If you want to, say, build the contents of a set of drop-down boxes, you're better off doing these one at a time. In your requested result set, you'd need to iterate through the dataset three times to do anything useful, and you would need to either check for NULL each time or needlessly iterate all the way to the end of the dataset.

If this is in a stored procedure, couldn't you run three separate SELECT DISTINCT and return the values as three results. Then you can consume them one at a time, which is what you would be doing anyway I would guess.

If there REALLY IS a connection between the values, you could add each of the results to an array or list, then access all three lists in parallel using the index.

Answer 3

Something like this maybe?

select *
from (
  select case 
          when row_number() over (partition by fruit_group) = 1 then fruit_group
          else null 
        end as fruit_group,
        case 
          when row_number() over (partition by name) = 1 then name
          else null
        end as name,
        case 
          when row_number() over (partition by caloric) = 1 then caloric
          else null
        end as caloric
  from your_table
) t
where fruit_group is not null
   or name is not null
   or caloric is not null

But I fail to see any sense in this