问SQL INSERT查询为什么将输入的值作为列名

EN

try
{
    Class.forName("com.mysql.jdbc.Driver");
    Connection con=DriverManager.getConnection("jdbc:mysql://localhost/db","root","123456");
    Statement s = con.createStatement();
    int start = 8, end = 10;
    char buf[] = new char[end-start];   // for extracting day alone.

    dvalue = new String();
    ddvalue = new String(); // for adding the extracted day to the table.
    dvalue = cb3.getSelectedItem().toString();
    dvalue.getChars(start, end, buf, 0);System.out.println(buf);
    ddvalue = String.copyValueOf(buf);


    s.executeUpdate("insert into "+nameoftab+" (sname,"+"_"+ddvalue+"_"+") values (hello,"+cb3.getSelectedItem()+")");
}
catch(SQLException s)
{
    System.out.println("SQL statemnet is not executed!");
    System.out.println(s);
}

s.executeUpdate("insert into "+nameoftab+" (sname,"+""+ddvalue+""+") values ("+cb5.getSelectedItem()+","+cb3.getSelectedItem()+")");