/* Test3.java 查看1年级1班共有多少学生 */ public static void main(String[] args) { try (Connection connection = DBUtil.getConnection();){ Statement statement = connection.createStatement(); String sql = "SELECT c.name,COUNT(s.id) FROM class c,student s WHERE s.class_id=c.id,AND c.name='1年级1班'"; ResultSet resultSet = statement.executeQuery(sql); while (resultSet.next()){ String cname = resultSet.getString("c.name"); int count = resultSet.getInt("COUNT(s.id)"); System.out.println(""+cname+","+count); } } catch (SQLException throwables) { throwables.printStackTrace(); } }改正该代码

代码存在两个问题：

1. SQL语句中逗号的位置不正确，应该是WHERE s.class_id=c.id AND c.name='1年级1班'。

2. 在输出结果时，应该将count值转换为字符串类型，否则会被当成加法运算。

修改后的代码如下：

/*         Test3.java         查看1年级1班共有多少学生      */     
public static void main(String[] args) {
    try (Connection connection = DBUtil.getConnection();) {
        Statement statement = connection.createStatement();
        String sql = "SELECT c.name,COUNT(s.id) FROM class c,student s WHERE s.class_id=c.id AND c.name='1年级1班'";
        ResultSet resultSet = statement.executeQuery(sql);
        while (resultSet.next()) {
            String cname = resultSet.getString("c.name");
            int count = resultSet.getInt("COUNT(s.id)");
            System.out.println("" + cname + "," + String.valueOf(count));
        }
    } catch (SQLException throwables) {
        throwables.printStackTrace();
    }
}