Select / group by multiple columns but count just the values of one column - sql

Question

I have 2 tables.
Table #1: orders
order_id | crit_1 | crit_2 | crit_3 | other
01 | A00 | GER | 49er | x
02 | A00 | GER | 49er | x
03 | A00 | USA | 49er | x
04 | C80 | DEN | 66er | x
05 | B50 | GER | 99er | x
The table orders has 3 important criteria but doesn't have the criterion_4. There is another table with the order_positions which contains multiple criterion_4 entries for each order_id.
Table #2: classifications
crit_1 | crit_2 | crit_3 | crit_4 | class_1 | class_2
A00 | GER | 49er | 4711 | A | 11
A00 | GER | 49er | 4712 | A | 21
A00 | USA | 49er | 4711 | D | 12
A00 | USA | 49er | 4712 | D | 21
B50 | GER | 99er | 4801 | B | 12
B50 | GER | 99er | 4802 | B | 12
B50 | GER | 99er | 4803 | B | 14
C80 | DEN | 66er | 4904 | C | 22
C80 | DEN | 66er | 4905 | C | 21
The table classifications contains classifications for:
orders = class_1 = combination of crit_1, crit_2 & crit_3
order_positions = class_2 = combination of crit_1, crit_2, crit_3
& crit_4
I have a query where I join classifications.class_1 on the table orders to create a list of all orders and their respective classification.
select
orders.order_id,
orders.crit_1,
orders.crit_2,
orders.crit_3,
classifications.class_1
from
orders
left join
classifications
on
orders.crit_1=classifications.crit_1 and
orders.crit_2=classifications.crit_2 and
orders.crit_3=classifications.crit_3
where
orders.others = "..."
group by
orders.order_id,
orders.crit_1,
orders.crit_2,
orders.crit_3,
classifications.class_1
I need a GROUP BY at the end since the table classifications contains multiple entries with the combination of crit_1, crit_2 and crit_3. But this isn't a problem since the needed classification_1 is always the same for each combination of crit_1, crit_2 and crit_3.
Now I want to create another query where I count just the number of each classification_1 for the orders. Something like this:
class_1 | number
A | 12
B | 5
C | 18
. | .
But I don't know how without the whole selection of orders.order_id, orders.crit_1, orders.crit_2, orders.crit_3 and classifications.class_1
I just want to count the class_1 classifications for the query above.
Any suggestions?
edit
I tried it like suggested by Kaushik Nayak:
select
--orders.order_id,
--orders.crit_1,
--orders.crit_2,
--orders.crit_3,
classifications.class_1,
count(*)
from
orders
left join
classifications
on
orders.crit_1=classifications.crit_1 and
orders.crit_2=classifications.crit_2 and
orders.crit_3=classifications.crit_3
where
orders.others = "..."
group by
--orders.order_id,
--orders.crit_1,
--orders.crit_2,
--orders.crit_3,
classifications.class_1
But the results are not correct and I have no idea how to reproduce those numbers.
A few examples:
| class_1 | query w/ | query w/o | query |
| | group by | group by | count(*) |
---------------------------------------------
| A | 654 | 2179 | 1024 |
| B | 371 | 1940 | 667 |
| C | 94 | 238 | 247 |
When I use my query with group by then I get 654 entries for class_1 = A.
When I make my query without group bythen I get 2179 entries for class_1= A.
And when I try the query with Count(*) then I get 1024 entries for class_1 = A.
The last one is definitely not correct.

Answer 1

Just use GROUP BY class_1 for your classifications table and add an EXISTS condition to check if there is an order.
SELECT
c.class_1,
COUNT(c.class_1) "number"
FROM
classifications c
WHERE
EXISTS (
SELECT
1
FROM
orders o
WHERE
o.crit_1 = c.crit_1
AND o.crit_2 = c.crit_2
AND o.crit_3 = c.crit_3
)
GROUP BY
c.class_1
ORDER BY
1;