MYSQL left join and inner join

Question

i have 3 tables

Employee => e_id (PK), dep_id (Fk)

Department => d_id (PK)

Attendance => id (PK), date_of_absence (DATE), e_id_f (FK)

i want to get the number of absent employees in certain department for a certain period.

i created a table called MyDates

CREATE TABLE MyDates (mydate date);

and this procedure to fill MyDates Table with Dates

CREATE PROCEDURE filldates(dateStart DATE, dateEnd DATE)

BEGIN

  WHILE dateStart <= dateEnd DO
    INSERT INTO MyDates (mydate) VALUES (dateStart);
    SET dateStart = date_add(dateStart, INTERVAL 1 DAY);
  END WHILE;
END;

then i called the procedure to fill MyDates with date CALL filldates('2017-01-01','2017-03-31');

Then i made this select statement :

select mydates.mydate, count(attendance.date_of_absence)
from mydates left join attendance on
mydates.mydate = attendance.date_of_absence
where mydates.mydate Between "2017-01-01" AND "2017-01-31"
group by mydates.mydate

This query gets what i need but for all departments, BUT for a certain department the number of rows is incorrect

select mydates.mydate, count(attendance.date_of_absence)
from mydates
left join attendance on mydates.mydate = attendance.date_of_absence
inner join employee on attendance.e_id_f = employee.e_id
where mydates.mydate Between "2017-01-01" AND "2017-01-31" AND employee.dep_id = 4
group by mydates.mydate;

This is a screenshot IMG

Answer 1

There are 3 issues with your query I can see. The 1st 2 results in dropping the dates where no matching record is found, the 3rd potentially means that you get wrong counts:

1. inner join employee on attendance.e_id_f = employee.e_id - if there is no matching record for a day, then attendance.e_id_f will be null for that day. The inner join will eliminate this record from the resultset. Solution: make this join a left as well.

2. AND employee.dep_id = 4 - this would only keep records in the resultset, that have an employee record associated with it, so if you have a day with 0 absences, this criteria would eliminate it from the resultset. Solution: include this criteria in the join condition.

3. count(attendance.date_of_absence) - counts occurances from the attendance table, which will not be correct after adding the 2nd left join. Solution: use count(employee.dep_id) instead.

Modified query:

select mydates.mydate, count(employee.dep_id)
from mydates
left join attendance on mydates.mydate = attendance.date_of_absence
left join employee on attendance.e_id_f = employee.e_id AND employee.dep_id = 4
where mydates.mydate Between "2017-01-01" AND "2017-01-31" group by mydates.mydate

Alternative solution is to use nested joins, where you specifically instruct MySQL to execute attendance inner join employee join first. You still need to move the employee.dep_id = 4 condition to the join condition:

select mydates.mydate, count(attendance.date_of_absence)
from mydates
left join (attendance inner join employee)
    on mydates.mydate = attendance.date_of_absence
       and attendance.e_id_f = employee.e_id
       and employee.dep_id = 4
where mydates.mydate Between "2017-01-01" AND "2017-01-31"
group by mydates.mydate

In the latter case the nested inner join ensures that only those attendance records are returned that belong to dep_id = 4 and these records are then left joined to your dates table. In this case there is no need to change the field you are counting.