Include zero counts when grouping by multiple columns and setting filters - sql

Question

I have a table (tbl) containing category (2 categories), impact (3 impacts), company name and date for example:
category | impact | company | date | number
---------+----------+---------+-----------|
Animal | Critical | A | 12/31/1999|1
Book | Critical | B | 12/31/2000|2
Animal | Minor | C | 12/31/2001|3
Book | Minor | D | 12/31/2002|4
Animal | Medium | E | 1/1/2003 |5
I want to get the count of records for each category and impact and be able to add rows with zero count and also be able to filter by company and date.
In the example result set below, the count result is 1 for category = Animal and company = A. The rest is 0 records and only the Critical and Medium impacts appear
category | impact | count
---------+----------+-------
Animal | Critical | 1
Animal | Medium | 0
I've looked at the responses to similar questions by using joins however, adding a WHERE clause doesn't include the zero records.
I also tried doing outer joins but it doesn't produce desired output. For example
select a.impact, b.category, ISNULL(count(b.impact), 0) from tbl a
left outer join tbl b
on b.number = a.number
and (a.category = 'Animal' and a.company in ('A'))
group by a.impact, b.category
produces
impact | category | count
---------+------------+--------
Medium | NULL | 0
Medium | Animal | 1
Critical | NULL | 0
Minor | NULL | 0
but the desired output should be
category | impact | count
---------+----------+-------
Animal | Critical | 1
Animal | Medium | 0
Animal | Minor | 0
Any help will be appreciated. Answers to associated questions don't have filtering so I will appreciate if someone can help with a query to produce desired output.

Answer 1

You need a master table with all the possible combinations of Categories and Impacts for this. Then Left join your table with the master and do the aggregation. Something like below
;WITH CAT
AS
(
SELECT
category
FROM Tbl
GROUP BY category
),
IMP
AS
(
SELECT
Impact
FROM Tbl
GROUP BY Impact
),MST
AS
(
SELECT
*
FROM CAT
CROSS JOIN IMP
)
SELECT
MST.category,
MST.Impact,
COUNT(T.Number)
FROM MST
LEFT JOIN Tbl T
ON MST.category = T.category
AND MST.Impact = T.Impact
AND T.Company = 'A'
WHERE MST.Category = 'Animal' GROUP BY MST.category,
MST.Impact