SQL: Combine result columns - sql

Question

SELECT Category, SUM (Volume) as Volume
FROM Product
GROUP BY Category;
The above query returns this result:
Category Volume
-------------------
Oth 2
Tv Kids 4
{null} 1
Humour 3
Tv 5
Theatrical 13
Doc 6
I want to combine some of the columns as one colum as follows:
Oth,{null}, Humour, Doc as Others
Tv Kids, Tv as TV
Theatrical as Film
So my result would look like:
Category Volume
-------------------
Others 12
Tv 9
Film 13
How would I go about this?

Answer 1

You need a CASE here, like this:
SELECT
CASE
WHEN Category IN ('Oth','Humour','Doc')
OR Category IS NULL THEN 'Others'
WHEN Category IN ('Tv Kids','Tv') THEN 'TV'
WHEN Category = 'Theatrical' THEN 'Film'
END as category ,
SUM (Volume) as Volume
from Product
GROUP BY
CASE
WHEN Category IN ('Oth','Humour','Doc')
OR Category IS NULL THEN 'Others'
WHEN Category IN ('Tv Kids','Tv') THEN 'TV'
WHEN Category = 'Theatrical' THEN 'Film'
END;
Null must be dealt with outside the IN list as it is a special value.

Answer 2

I think you need to use a case statement to group categories together.
select case category when 'Tv' then 'Tv'
when 'Film' then 'Film'
else 'Other'
end as Category,
sum(Volume) as Volume
from (
SELECT Category, SUM (Volume) as Volume
FROM Product
GROUP BY Category
) subcategoryTotals
group by Category
(I think most DBs will allow you to group by the alias Category. (If not you can re-use the case statement)
Edit: Just a final thought (or two):
You should consider normalizing your database - for example, the Category column should really be a foreign key to a Categories table.
Also, this sql is reasonably ok because the case statement isn't too long or complex. If you wanted to split things up further it could quickly get to be unmanageable. I'd be inclined to use the idea of categories and subcategories in my database.

Answer 3

The best solution might be to implement those groups in the database. For instance:
category_group
id_category_group name sortkey
1 Others 3
2 TV 2
3 Film 1
category
id_category name id_category_group
1 Oth 1
2 Tv Kids 2
3 Humour 1
4 Tv 2
5 Theatrical 3
6 Doc 1
query
SELECT g.Name, SUM (p.Volume) as Volume
FROM Product p
LEFT JOIN Category c ON c.Id_Category = p.Id_Category
LEFT JOIN Category_Group g ON g.Id_Category_Group = c.Id_Category_Group
GROUP BY g.Id_Category_Group, g.Name
ORDER BY g.sortkey;
This makes NULL a group of its own, though. But well, it is a group of its own, as NULL means not known (yet), so you don't actually know whether it's TV, Film or Other. If you still want to count NULL as Others, change the ON clause accordingly:
LEFT JOIN Category_Group g
ON g.Id_Category_Group = COALESCE(c.Id_Category_Group, 3) -- default to group 'Others'

Answer 4

Try following,
select category_group , sum(volume) as Volume from
(
SELECT
Category,
Volume,
case
WHEN Category IN ('Oth','Humour','Doc','{null}') THEN 'Others'
WHEN Category IN ('Tv Kids','Tv') THEN 'TV'
WHEN Category = 'Theatrical' THEN 'Film'
end as category_group
FROM Product
) T
group by category_group