postgresql RIGHT Join: limit returned rows - postgresql

Question

I have the following schema:
expenses
id
name, varchar
cost, double
date, DATE
category_id, int f_key
user_id, int f_key
1
Pizza
22.9
22/08/2022
1
1
2
Pool
34.9
23/08/2022
2
1
categories
id
name, varchar
1
Food
2
Leisure
3
Medicine
4
Fancy food
users_categories(user_id int foreign key, category_id foreign key)
user_id int f_key
category_id int f_key
1
1
1
2
1
3
2
4
And two users with id 1 and 2.
Relation between user and category is many to many.
Problem:
I want to get statistics (total cost amount and count) for all categories. For categories where there are no expenses I want to return 0. Here is my query:
SELECT categories.name as name, count(expenses.name) as count, round(SUM(price)::numeric,2) as sum
FROM expenses
Right JOIN categories ON expenses.category_id = categories.id
and expenses.category_id in (
select users_categories.category_id from users_categories where users_categories.user_id = 1
)
and expenses.id in(
Select expenses.id from expenses
join users_categories on expenses.category_id = users_categories.category_id
and expenses.user_id = 1
AND (extract(year from date) = 2022 OR CAST(2022 AS int) is null)
AND (extract(month from date) = 8 OR CAST(8 AS int) is null)
)
GROUP BY categories.id ORDER BY categories.id
The response is:
name
count
sum
Food
1
22.9
Leisure
1
33.9
Medicine
0
null
Fancy food
0
null
How I should edit my query to eliminate the last row, because this category doesn't belong to the user 1.

Answer 1

In your query you used user_categories as subquery so it will not filter category ids,
Try this Query
SELECT categories.name as name,count(expenses.name) as count, coalesce(round(SUM(price)::numeric,2),0) as sum from
categories
left join users_categories on users_categories.category_id= categories.id
left join expenses ON expenses.category_id = categories.id
AND (extract(year from date) = 2022 OR CAST(2022 AS int) is null)
AND (extract(month from date) = 8 OR CAST(8 AS int) is null)
where users_categories.user_id='1'
GROUP BY categories.name,categories.id ORDER BY categories.id
OUTPUT :
name count sum
Food 1 22.90
Leisure 1 34.90
Medicine 0 0

Answer 2

You want to move expenses.category_id in ... out of the ON condition and into a WHERE clause.
When it is in the ON clause, that means rows which were removed by the in-test just get NULL-fabricated anyway. You want to remove those rows after the NULL-fabrication is done, so that they remain removed. But why do you use that in-test anyway? Seems like it would be much simpler written as another join.

Answer 3

What I understand is that you are trying to get the count and sum of expenses for all the categories related to the user_id 1 within the month of august 2022.
Please try out the following query.
WITH statistics
AS (SELECT e.category_id,
Count(e.*) AS count,
Round(Sum(e.cost), 2) AS sum
FROM expenses e
WHERE e.user_id = 1
AND ( e.date BETWEEN '01/08/2022' AND '31/08/2022' )
GROUP BY e.category_id),
user_category
AS (SELECT uc.category_id,
COALESCE(s.count, 0) AS count,
COALESCE(s.sum, 0) AS sum
FROM users_categories uc
LEFT JOIN statistics s
ON uc.category_id = s.id
WHERE uc.user_id = 1)
SELECT c.NAME,
u.count,
u.sum
FROM categories c
INNER JOIN user_category u
ON u.category_id = c.id;