Using CASE to properly count items with if/else logic in SQL - sql

Question

Say I have a table table in the form:
| user | class |
|------|-------|
| 1 | a |
| 1 | b |
| 1 | b |
| 2 | b |
| 3 | a |
There are only two classes.
I want to write a query such that we count the number of users in each class such that any user who has label a and b gets sorted into a, any user with just a gets sorted into a and then any user with just b gets into b. If applied to the table snippet above we would get:
| class | count |
|-------|-------|
| a | 2 |
| b | 1 |
Also acceptable is the transpose, like:
| a | b |
|---|---|
| 2 | 1 |
My current solution involves two CTEs:
WITH a_users AS
(
SELECT
user,
SUM(CASE WHEN class = 'a' THEN 1 ELSE 0 END) AS a_class
FROM
table
WHERE
class in ('a', 'b')
GROUP BY
user
),
labeled_users as (
SELECT
user,
CASE WHEN a_class >=1 then 'a' ELSE 'b' END as label
FROM
a_users
)
SELECT
label,
COUNT(DISTINCT user)
FROM
labeled_users;
Is there a (1) more efficient way to solve for this or (2) a more concise/readable solution?

Answer 1

Basically, you want "a" for a user who has "a" at all. A subquery is the first approach:
select sum(case when num_as > 0 then 1 else 0 end) as num_class_a,
sum(case when num_as = 0 then 1 else 0 end) as num_class_b
from (select user, sum(case when class = 'a' then 1 else 0 end) as num_as
from t
group by user
) t;
With a little trick, you can eliminate the subquery:
select count(distinct case when class = 'a' then user end) as num_as,
count(distinct user) - count(distinct case when class = 'a' then user end) as num_bs
from t;

Answer 2

Something like this should work, if a and b really are your classes. Otherwise adjust the min/max as needed.
; with CTE as (
Select user, min(class) as Class
from Labeled_Users
group by user)
Select Class, count(*)
from CTE
group by Class

Answer 3

Here is a straight forward query to get the job done using a subquery and conditional aggregation. It should return the second version of your expected result (pivoted) :
SELECT
SUM(CASE WHEN x.minc <> x.maxc OR x.maxc = 'a' THEN 1 ELSE 0 END) a,
SUM(CASE WHEN x.minc = x.maxc AND x.maxc = 'b' THEN 1 ELSE 0 END) b
FROM (
SELECT user, MAX(class) maxclass, MIN(class) minclass
FROM mytable
GROUP BY user
) x
The subquery computes the minimum and maximum class of each user. Then the outer query separatly counts users :
a : users that belong to both classes or just to class a
b : users that belong to class b only
This is standard SQL syntax that will work on most RDBMS (obviously, even those who do not support CTEs, such as pre-8.0 MySQL versions).

Answer 4

Using String_agg():
with usr_class as(
SELECT DISTINCT usr,
string_agg(txt,':') as all_class
FROM abc
GROUP BY usr
)
select count(usr),
case when POSITION('a' in all_class)>0 THEN 'a'
ELSE 'b'
END AS CLASS
FROM usr_class
GROUP BY case when POSITION('a' in all_class)>0 THEN 'a'
ELSE 'b'
END;