Combine multiple counts with group by including zeros - sql

Question

I have 3 tables: Equipment, Sections and Users and i want to combine 3 tables grouped by section and show total equipment per section and total users per section including zeros
Example equipment table:
ID Device Name SectionID Device Type
1 Holly's computer 1 PC
2 John's mobile 2 mobile
3 Maggie's printer 3 printer
4 Jonathan's scanner 3 scanner
5 George's scanner 2 scanner
6 Dugans handheld 5 scanner
7 Main printer 5 printer
Example sections table:
ID Section
1 Finance
2 HR
3 IT
4 Manager
5 Storage
Example users table
ID User SectionID
1 John 3
2 George 2
3 Amanda 2
4 Andy 4
5 Katherine 1
I tried this:
SELECT
b.section AS GROUPED_SECTION,
COUNT(distinct a.sectionid) AS TOTAL_EQUIPMENT,
COUNT(distinct c.sectionid) AS TOTAL_USERS
FROM Equipment a
LEFT JOIN Section b ON a.sectionid=b.id
LEFT JOIN Users c on a.sectionid=c.sectionid
GROUP BY b.description
ORDER BY b.description
but something is not working correctly
I want to create a query that will have the following result:
SECTION TOTAL_EQUIPMENT TOTAL_USERS
------- --------------- ------------
Finance 1 1
IT 2 1
HR 2 2
Manager 0 1
Storage 2 0
-1st column presents distinct sections from Equipment table
-2nd column presents total equipment per section according to Equipment table
-3rd column presents total users under that section according to Users table

Answer 1

Using UNION ALL for SectionID on equipment and users tables. and make a grp column to split two different tables.
then do OUTER JOIN base on sections table, final use aggregate function get count.
Query 1:
SELECT s.Section,
COUNT(CASE WHEN grp = 1 THEN 1 END) TOTAL_EQUIPMENT,
COUNT(CASE WHEN grp = 2 THEN 1 END) TOTAL_USERS
FROM sections s
LEFT JOIN (
select SectionID,1 grp
from equipment
UNION ALL
select SectionID,2 grp
from users
) t1 on t1.SectionID = s.ID
GROUP BY s.Section
Results:
| Section | TOTAL_EQUIPMENT | TOTAL_USERS |
|---------|-----------------|-------------|
| Finance | 1 | 1 |
| HR | 2 | 2 |
| IT | 2 | 1 |
| Manager | 0 | 1 |
| Storage | 2 | 0 |

Answer 2

The query is straightforward with CTE, count each of the two child tables, then join them to parent table (section)
Live test: http://sqlfiddle.com/#!18/f9f99/4
with eq as
(
select sectionid, count(*) as total_equipment
from equipment
group by sectionid
)
, uq as
(
select sectionid, count(*) as total_users
from users
group by sectionid
)
select s.id, s.section,
total_equipment = isnull(eq.total_equipment, 0),
total_users = isnull(uq.total_users, 0)
from sections s
left join eq on s.id = eq.sectionid
left join uq on s.id = uq.sectionid
order by s.section
Output:
| id | section | total_equipment | total_users |
|----|---------|-----------------|-------------|
| 1 | Finance | 1 | 1 |
| 2 | HR | 2 | 2 |
| 3 | IT | 2 | 1 |
| 4 | Manager | 0 | 1 |
| 5 | Storage | 2 | 0 |