A simple doubt in Servlets related to ResultSet

[abctech]

My requirement is such that any user must not be allowed to save the same record twice.

For this what I do is as soon as some user fills up the page and clicks 'Save' I invoke a Servlet which checks the information filled in that page with that of the existing records and if a match is found then a message is printed back saying 'record already exists,cannot save twice!'.

I am using PreparedStatement to execute a MySql query in my Servlet and an if-else block to distinguish between an existing record and a fresh record,roughly my code is something like this:-

String s = request.getParameter("StudentID");
String s1 = request.getParameter("SubjectID");
 
PreparedStatement pst = con.prepareStatement(select * from abc where (studid=? && subid=?);); //Note: I dont want my table abc to have a duplicate studid-subid combination, one student can study only one subject at a time.
 
pst.setInt(1,Integer.parseInt(s));
pst.setInt(2,Integer.parseInt(s1));
 
ResultSet rs = null;
rs = pst.executeQuery();
 
if(rs != null)
{
    //means the record already exists in table 'abc' so dont save this record twice,instead alert the user'This Record already exists!'    
}
else
{
   //since ResultSet is null that means no matching record was found in table'abc' indicating that the user has entered a fresh record so carry on the  processing...
}

Now I'm guessing that if the user-entered-studid+subid combination is a fresh one then 'rs' should be null since the query isn't fetching any matching combinations..but still the control executes the if-part.

My question is if some query doesnt yield a result then shouldnt the Resultset associated with it be null or perhaps empty?Is there a way of checking that?

..or should I rather check in a while-loop[while(rs.next())] instead of saying if(rs!=null)?

Please guide ..Thanks in advance!

[r035198x]

My requirement is such that any user must not be allowed to save the same record twice.

For this what I do is as soon as some user fills up the page and clicks 'Save' I invoke a Servlet which checks the information filled in that page with that of the existing records and if a match is found then a message is printed back saying 'record already exists,cannot save twice!'.

I am using PreparedStatement to execute a MySql query in my Servlet and an if-else block to distinguish between an existing record and a fresh record,roughly my code is something like this:-

String s = request.getParameter("StudentID");
String s1 = request.getParameter("SubjectID");
 
PreparedStatement pst = con.prepareStatement(select * from abc where (studid=? && subid=?);); //Note: I dont want my table abc to have a duplicate studid-subid combination, one student can study only one subject at a time.
 
pst.setInt(1,Integer.parseInt(s));
pst.setInt(2,Integer.parseInt(s1));
 
ResultSet rs = null;
rs = pst.executeQuery();
 
if(rs != null)
{
//means the record already exists in table 'abc' so dont save this record twice,instead alert the user'This Record already exists!'    
}
else
{
//since ResultSet is null that means no matching record was found in table'abc' indicating that the user has entered a fresh record so carry on the processing...
}

Now I'm guessing that if the user-entered-studid+subid combination is a fresh one then 'rs' should be null since the query isn't fetching any matching combinations..but still the control executes the if-part.

My question is if some query doesnt yield a result then shouldnt the Resultset associated with it be null or perhaps empty?Is there a way of checking that?

..or should I rather check in a while-loop[while(rs.next())] instead of saying if(rs!=null)?

Please guide ..Thanks in advance!

Resultset returned is never null. See here.

[abctech]

Resultset returned is never null. See here.

Yes r0, I got it now, Thanks!
I should have gone through the documentation properly!

[r035198x]

Yes r0, I got it now, Thanks!

I should have gone through the documentation properly!

Sometimes it is easy to overlook the docs. Most of the problems posted here are solved by a check on the docs.

[abctech]

Sometimes it is easy to overlook the docs. Most of the problems posted here are solved by a check on the docs.

Yeah,thts true :)

[dmjpro]

u should use resultset.next() if it return false...then no records exist
else match found...................

[abctech]

u should use resultset.next() if it return false...then no records exist
else match found...................

Yes,I referred the docs for the methods that I could apply here,its working now,but thanks for your response,

Cheers!