I'm having trouble with COUNT() values when joining tables SQL - sql

Question

I have two independent tables, tbl_timesheet and tbl_absence. tbl_timesheet will have a row every day that an employee logs into a system. tbl_absence is a single row for a unique instance of absence, where the employee isn't in work. Each table looks like:
tbl_timesheet:
Staff_ID DEPT LOG_DATE
001 IT 2020-09-01
002 HR 2020-09-01
003 SALES 2020-09-01
001 IT 2020-09-02
002 HR 2020-09-02
003 SALES 2020-09-02
001 IT 2020-09-03
002 HR 2020-09-03
003 SALES 2020-09-03
tbl_absence:
Staff_ID ABSENCE_DATE
001 2020-09-10
003 2020-09-15
003 2020-09-22
I want to join the two tables, where I can count the instances of absence. I've attempted to do this using the following script:
SELECT t.Staff_ID as ID, t.DEPT as Dept, COUNT(a.Staff_ID) as 'Instances'
FROM tbl_timesheet t
JOIN tbl.absence a
ON t.Staff_ID = a.Staff_ID
GROUP BY t.Staff_ID, t.DEPT
I'd expect the following:
ID Dept Instances
001 IT 1
003 SALES 2
However due to the join between the tables, I believe the Staff_ID is being duplicated because each appears multiple times in tbl_timesheet.
Any suggestions?

Answer 1

when you JOIN two tables before getting distinct values of Staff_Id and Dept it will multiply the counts of records. for example staff_id='003' 2 record from absence table multiply 3 records from timesheet and you will get 6 records of it.Therefore you can code as below.
SELECT
t.Staff_ID as ID,
t.DEPT as Dept,
-----------
COUNT(a.Staff_ID) as Instances
-----------
FROM tbl_absence a
JOIN (select distinct Staff_ID, DEPT FROM tbl_timesheet) t
ON t.Staff_ID = a.Staff_ID
GROUP BY t.Staff_ID, t.DEPT