Question

I am storing employee attendance on a table ‘attendance’ having the following structure:

EmpID, CDate

Attendance system insert this table everyday with employee-id of all employees present on that particular day.

I need to find out absent statement of a particular employee. I can do this easily by selecting all distinct date that are not in – dates where the employee is present.

Is there any way I can remove the not in operator on that sql statement. Please help

Here is the sql query for employee with EmpId 01:

select distinct CDate 
  from attendance 
  where CDate not in (
      Select CDate from attendance where EmpID='01') 

Answer 1

The problem isn’t in the NOT IN clause, it is the subquery.

You want something more like:

SELECT DISTINCT a1.CDate, if (EmpID=NULL, false,true) as Present
FROM attendance as a1
LEFT JOIN attendance as a2 USING (CDate) 
WHERE a2.EmpID='01'

This is a cartesian join which pulls all of the dates, then joins the employee attendance status on that. Should be significantly faster than your subquery.

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Updated, with tested code:

SELECT DISTINCT a1.CDate, IF (a2.EmpID IS NULL, false,true) as Present
FROM attendance AS a1
LEFT JOIN attendance AS a2 ON (a1.CDate = a2.CDate AND a2.EmpID='01') 

My bad on the previous answer. I should have put the subselection into the ON instead of an aggregate.