结果集在Mysql中不起作用

Question

我有一张这样的表

注册表

Login_id   Address  User_type  state 

    1      Ansari     s        Alaska
    2      Rajesh     b        Rameshwaram

String retrieve_query="select User_type from Signup where State="Alaska";                           
ResultSet rs1=st.executeQuery(retrieve_query);

String User_type=rs1.getString("User_type");//I expect "S" to be the User_type

但我得到以下异常

发生异常java.sql.SQLException：ResultSet关闭后不允许操作

Answer 1

你错过了rs1.next()。

String User_type=null;
if (rs1.next()) 
    User_type = rs1.getString("User_type");

Answer 2

您应该尝试使用prepared statement，如下所示：

PreparedStatement ps;
ResultSet rs;
String user_type=null;
ps = con.prepareStatement("select User_type from Signup where State='?'");
ps.setString(1, "Alaska");
rs = ps.executeQuery();
if (rs.next()) 
   user_type = rs.getString("User_type");

Answer 3

尝试用以下内容替换您的查询：

String retrieve_query="select User_type from Signup where State='Alaska'";