Filter rows to return the exact relationship

I have two tables, expenses and categories, they have a many-to-many relationship through the table expenses_categories. I'm trying to implement a filter by categories, lets say that I provided the id for the categories A and B, I want to return the expenses who only have A and B. For example:

Expense X have Category A, B, and C
Expense Y have Category A and B
Expense Z have Category B

I want to return only the Expense Y

I'm using PostgreSQL by the way. I really need to learn how to do this kind of stuff.

Categories

ID	NAME
1	TV
2	CC
3	NET

ExpensesCategories

expense_id	category_id
1	1
1	2
2	1
2	2
2	3
3	1
4	2

I want to get all the Expenses that have ONLY the Categories 1 and 2.
In that case, I expect to only get the Expense 1

expense_id	category_id
1	1
1	2

Solution

You can group by expense_id and use STRING_AGG() in the HAVING clause to collect all the category_ids of each expense_id and compare it to a string like '1,2' which contains the category_ids that you want in ascending order as a comma separated list:

SELECT expense_id
FROM ExpensesCategories
GROUP BY expense_id
HAVING STRING_AGG(category_id::text, ',' ORDER BY category_id) = '1,2';

If you want all the rows of these expense_ids in ExpensesCategories, use the above query as a CTE:

WITH cte AS (
  SELECT expense_id
  FROM ExpensesCategories
  GROUP BY expense_id
  HAVING STRING_AGG(category_id::text, ',' ORDER BY category_id) = '1,2'
)
SELECT * 
FROM ExpensesCategories
WHERE expense_id IN (SELECT expense_id FROM cte);

See the demo.