Select from one table with count from two other tables - mysql

Question

I'm doing a query where i select all from a table called employee and want to do a count of employee_id from two other tables and represent the count in 2 seperate columns.
The tables:
employee [id, etc.]
report [id, employee_id, etc.]
office_report[id, employee_id, etc.]
What i did so far is:
SELECT emp.*, COUNT(rep.id ) no_of_field_reports, COUNT(of_rep.id) no_of_office_reports
FROM employee emp
LEFT JOIN report rep
ON (emp.id = rep.employee_id)
LEFT JOIN office_report of_rep
ON (emp.id = of_rep.employee_id)
WHERE emp.user_id =7 AND emp.active = 1
GROUP BY emp.id, emp.name
ORDER BY emp.name ASC
The problem is, as soon as i have reports in BOTH report tables the count messes up. Say i have 16 reports in report table and 2 in office_report table, the count for no_of_field_reports and no_of_office_reports will become 32.
Im missing something obviously but as I'm not a SQL genius I can't figure out what.
Please make sure to explain what is causing the problem so I'm able to learn from my mistakes and get a better understanding of these type of queries as this is not going to be the last time.
I guess the answer will be the same for mariaDB, mySQL, and SQL in general so i added all those tag's for the sake of attention..

Answer 1

Possibly one approach if you're after distinct counts ( though you may need to adjust to the PK field)
SELECT emp.*,
COUNT(distinct rep.id ) no_of_field_reports, --may need to be on Unique key instead
COUNT(distinct of_rep.id) no_of_office_reports --may need to be on Unique key instead)
FROM employee emp
LEFT JOIN report rep
ON (emp.id = rep.employee_id)
LEFT JOIN office_report of_rep
ON (emp.id = of_rep.employee_id)
WHERE emp.user_id =7 AND emp.active = 1
GROUP BY emp.id, emp.name
ORDER BY emp.name ASC
An approach getting the counts before the joins if you're not after a distinct count then this is likely the right approach and offers flexibility.
SELECT emp.*, rep.cnt, of_Rep.cnt
FROM employee emp
LEFT JOIN (SELECT count(ID) cnt , employee_ID
FROM REPORT
GROUP BY employee_ID) rep
ON (emp.id = rep.employee_id)
LEFT JOIN (SELECT count(ID) cnt, Employee_ID
FROM office_report
GROUP BY employee_ID) of_rep
ON (emp.id = of_Rep.employee_id)
WHERE emp.user_id =7 AND emp.active = 1
GROUP BY emp.id, emp.name
ORDER BY emp.name ASC
or use of correlated queries (but not supported all the time Such as when creating materialized views from this SQL)
SELECT emp.*,
(SELECT count(ID)
FROM REPORT
WHERE emp.id = rep.employee_id) Report_Cnt,
(SELECT count(ID)
FROM office_report of_REP
WHERE emp.id = of_Rep.employee_id) of_Rep_Cnt
FROM employee emp
WHERE emp.user_id =7 AND emp.active = 1
GROUP BY emp.id, emp.name
ORDER BY emp.name ASC