Counting SQLite rows that might match multiple times in a single query - sql

Question

I have a SQLite table which has a column containing categories that each row may fall into. Each row has a unique ID, but may fall into zero, one, or more categories, for example:
|-------+-------|
| name | cats |
|-------+-------|
| xyzzy | a b c |
| plugh | b |
| quux | |
| quuux | a c |
|-------+-------|
I'd like to obtain counts of how many items are in each category. In other words, output like this:
|------------+-------|
| categories | total |
|------------+-------|
| a | 2 |
| b | 2 |
| c | 2 |
| none | 1 |
|------------+-------|
I tried to use the case statement like this:
select case
when cats like "%a%" then 'a'
when cats like "%b%" then 'b'
when cats like "%c%" then 'c'
else 'none'
end as categories,
count(*)
from test
group by categories
But the problem is this only counts each row once, so it can't handle multiple categories. You then get this output instead:
|------------+-------|
| categories | total |
|------------+-------|
| a | 2 |
| b | 1 |
| none | 1 |
|------------+-------|
One possibility is to use as many union statements as you have categories:
select case
when cats like "%a%" then 'a'
end as categories, count(*)
from test
group by categories
union
select case
when cats like "%b%" then 'b'
end as categories, count(*)
from test
group by categories
union
...
but this seems really ugly and the opposite of DRY.
Is there a better way?

Answer 1

Fix your data structure! You should have a table with one row per name and per category:
create table nameCategories (
name varchar(255),
category varchar(255)
);
Then your query would be easy:
select category, count(*)
from namecategories
group by category;
Why is your data structure bad? Here are some reasons:
A column should contain a single value.
SQL has pretty lousy string functionality.
SQL queries to do what you want cannot be optimized.
SQL has a great data structure for storing lists. It is called a table, not a string.
With that in mind, here is one brute force method for doing what you want:
with categories as (
select 'a' as category union all
select 'b' union all
. . .
)
select c.category, count(t.category)
from categories c left join
test t
on ' ' || t.categories || ' ' like '% ' || c.category || ' %'
group by c.category;
If you already have a table of valid categories, then the CTE is not needed.