如何从选择查询中获取另一个字符串？

Question

I want to get string from column no. 4 from my database to check user privileges.

Can I use rs.getString(index) to get data from column no.4? I want to check user´s privileges...so if the column data is equal 4, the page will be redirected to AdminControlPanel.jsp BUT, this code doesn´t work :(

String user=request.getParameter("login");
String pass=request.getParameter("password");
 Class.forName("com.mysql.jdbc.Driver").newInstance();
    Connection con=DriverManager.getConnection("jdbc:mysql://localhost:3306/android","root","root");  
           Statement st=con.createStatement();
           ResultSet rs=st.executeQuery("select * from users where login='"+user+"' and password='"+pass+"'");
           String p = rs.getString(4);                   
           int count=0;
           while(rs.next()){
           count++;
          }
      if(count>0 && p == "4"){
             // out.println(rs);
                           response.sendRedirect("AdminControlPanel.jsp");
                      }
          else{
               out.println("aaa");
               response.sendRedirect("#");
           }
        }
                 catch(Exception e){
    System.out.println(e);
}

Answer 1

You want

while (rs.next()) {
   String val = rs.getString(4);
   ....

Note that iterating through a ResultSet iterates through the rows . For each row, the column indexing starts from '1'.

However it's safer to get by column name, since your SQL query doesn't specify neither the columns nor the order in which they're returned:

String val = rs.getString("COLUMN_NAME");

I see from the below that you need an integer. Check out the doc for ResultSet for more info, but:

int val = rs.getInt("COLUMN_NAME");

As an aside, I don't see you closing your ResultSet / Statement / Connection in the above. If you're not, then you'll need to!

Answer 2

String p = rs.getString(4);   // This should be inside your while                  
int count=0;
while(rs.next()){
    count++;
}

• You should move your first line inside your while loop. You can't fetch the columns of a row, until you move your cursor to that row using res.next() .

• Also, since your database should ideally have only one record for a combination of username and password . So, you can better use an if instead of while .

• And you don't really need a count variable there.

So, your code should be: -

ResultSet rs=st.executeQuery("select * from users where login='"+user+"' " +
                              "and password='"+pass+"'");
if (rs.next()) {
    String p = rs.getString(4);  // Note that using Column name is a better idea

    // or rs.getInt(4) if the column type is `int`

    if(p.equals("4")) {   // Use equals method to compare string content
         response.sendRedirect("AdminControlPanel.jsp");

    } else{
         out.println("aaa");
         response.sendRedirect("#");
    }
}

Also, note that you should compare your string using equals method. if (p == "4") will give you false result. == operator does not compare the content of the string, rather the content of the reference used in comparison.

Answer 3

you are comparing two String objects rather than checking the values in the String.

just change the code to p.equals("4") and try.