Selecting distinct values for multiple columns - sql

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