JDBC COUNT（）如何获得结果行

Question

我对JDBC非常陌生，我正在练习执行查询和语句。

try{
            Connection conn = DriverManager.getConnection(DB_URL, USERNAME, PASSWORD);

            Statement st = conn.createStatement();

            String queary = "SELECT COUNT(*) AS \"Number of suplliers\", city\r\n" + 
                    "FROM supplier\r\n" + 
                    "GROUP BY city\r\n" + 
                    "ORDER BY city DESC\r\n";
            //st.executeUpdate(queary);

            ResultSet rs = st.executeQuery(queary); 

            while(rs.next()) {
                System.out.println(rs.getString("supplierNum"));
                System.out.println(rs.getString("city"));
                space();
            }   

        st.close();
        conn.close();

        }

我正在尝试使用SELECT COUNT(supplierNum) AS "Number of suplliers", cityFROM supplierGROUP BY city ORDER BY city DESC;执行并打印结果

控制台刚刚终止，没有任何反应，没有错误消息，只是一个空白屏幕。

这是我要访问的表

CREATE TABLE supplier (
  supplierNum CHAR(2)   NOT NULL,
  name CHAR(10) NOT NULL,
  status TINYINT(4) NOT NULL,
  city VARCHAR(10)  NOT NULL,
   PRIMARY KEY (supplierNum)
);

Answer 1

您将count列别名为"Number of suplliers"，但是您的JDBC代码正在尝试查找supplierNum。在各处使用一致的别名来解决此问题：

String query = "SELECT city, COUNT(*) AS supplierCnt " + 
               "FROM supplier " + 
               "GROUP BY city " + 
               "ORDER BY city DESC";

ResultSet rs = st.executeQuery(queary); 

while (rs.next()) {
    System.out.println(rs.getString("supplierCnt"));
    System.out.println(rs.getString("city"));
    space();
}