ExecuteUpdate error with mysql

Question

Dear All:
i have a wierd error right here, the code is working correctly for the first call to this jsp page but the error come out when second call to this page. the field was update wrongly, credit=credit-1 become credit=credit-2 and Point=Point+1 become Point=Point+2.
I am using the mysql and tomcat
<%
String a1="update studentdetail Set credit=credit-1 where UserID = '"+UserID+"'";//deduct from student
a=statement.executeUpdate(a1);
out.println(a1+" "+a);
String a2="update teacherinfo Set Point=Point+1 where TeacherID = '"+tID+"'";//add into teacherinfo table
a=statement.executeUpdate(a2);
out.println(a2+" "+a);
String a3="update teachersubcription Set Point=Point+1 where ID = '"+myID+"'";//add into teacher subcription table
a=statement.executeUpdate(a3);
out.println(a3+" "+a);
%>
please help

Answer 1

It's hard to tell from the code you provided, but it sounds like a classic case of not reinitializing variables. Depending on where this code lives and how it gets used, you'll update your database with credit-1 and that credit-1 value which now lives in your DB must be finding it's way back into the credit variable. Check everyplace you set that variable and everyplace you read the DB and I'll bet you find your answer.
Cheers