That's still not much information to solve the problem... But here are some ideas, of what MIGHT go wrong.

First, don't use ctod() in the comparisons. Use unambiguous date constants, in this case, {^2010-06-20}

Second, please note that the way you are combining conditions with AND, all the conditions must be fulfilled for the record to be included. Check whether you have records that fulfill those conditions.

>>Perhaps you should explain what is the difference between the expected result, and the result you actually get?
>>
>>>I have three queries, then i joint. but not correct result.
>>>
>>>

>>>
>>>SELECT  Hwsv.year,Hwsv.scode,Hwsv.lcode,Hwsv.sec, Hwsv.perc,COUNT(*)as nowork,;
>>>  Student.sex;
>>> FROM ;
>>>     student ;
>>> left JOIN hwsv ;
>>>   ON  Student.code = Hwsv.code;
>>>    WHERE  Hwsv.year=thisform.year.value AND hwsv.scode=thisform.scoDE.Value and Hwsv.perc<50 AND student.admtd=1 AND student.joind<CTOD("20/06/2010") ;
>>>              ORDER BY hwsv.lcode;
>>>              GROUP BY hwsv.lcode,hwsv.sec;
>>>              INTO cursor nowork
>>>  
>>> brows
>>>    SELECT Hwsv.year,Hwsv.scode,Hwsv.lcode,Hwsv.sec, Hwsv.perc,COUNT(*) as completee,;
>>>  Student.sex;
>>> FROM ;
>>>     student ;
>>> left    JOIN hwsv ;
>>>   ON  Student.code = Hwsv.code;
>>>    WHERE  Hwsv.year=thisform.year.value AND hwsv.scode=thisform.scoDE.Value AND Hwsv.perc =100 and student.admtd=1 AND student.joind<CTOD("20/06/2010");
>>>              GROUP BY hwsv.lcode,hwsv.sec;
>>>       ORDER BY hwsv.lcode;
>>>             INTO cursor com
>>>                 brows
>>>      
>>>      
>>>      
>>>       SELECT Hwsv.year,Hwsv.scode,Hwsv.lcode,Hwsv.sec, Hwsv.perc,COUNT(*) as incompl ,;
>>>  Student.sex;
>>> FROM ;
>>>     student ;
>>>    left JOIN hwsv ;
>>>   ON  Student.code = Hwsv.code;
>>>    WHERE  Hwsv.year=thisform.year.value AND hwsv.scode=thisform.scoDE.Value AND Hwsv.perc <100 and Hwsv.perc>49 AND student.admtd=1 AND student.joind<CTOD("20/06/2010");
>>>              GROUP BY hwsv.lcode,hwsv.sec;
>>>       ORDER BY hwsv.lcode;
>>>       INTO CURSOR incom
>>>
>>>
>>>SELECT Com.*, Incom.*, Nowork.*;
>>> FROM ;
>>>     com ;
>>>   LEFT JOIN incom ;
>>>   ON  Com.scode = Incom.scode;
>>>   AND  Com.lcode = Incom.lcode;
>>>   AND  Com.sec = Incom.sec ;
>>>    left JOIN nowork ;
>>>   ON  Com.scode = Nowork.scode;
>>>   AND  Incom.lcode = Nowork.lcode;
>>>   AND  Incom.sec = Nowork.sec;
>>>       INTO CURSOR  ghu10cc
>>>

>some result is missing from first query