Question

编辑：oops。这些JDBC语句有效;我忘记了在SQL Plus中提交。谢谢保罗。

当我使用SQL Plus查询数据库时：

select count(tid) from retweets where tid = 35          => 2
select count(tid) from tweets where replyto = 35        => 1

我尝试了几种方法通过JDBC从数据库中提取这些聚合计数，但在所有情况下，他们都返回0。

示例：

Statement stmt = m_con.createStatement();
ResultSet retweets = stmt.executeQuery("select count(tid) from retweets where tid = 35");

if (retweets.next()) {  System.out.println("# of Retweets: " + retweets.getInt(1));}
 ResultSet replies = stmt.executeQuery("select count(tid) Replies from tweets where replyto = "+tid);


if (replies.next()) {System.out.println("# of Replies : " + replies.getInt("Replies"));}

两次都打印了0。为什么会发生这种情况，我该如何解决？感谢。

Answer 1

这样的事情：

public class TweetDao {
    private static final String SELECT_TWEETS = "SELECT COUNT(tid) as TC FROM TWEETS WHERE replyTo =  ? ";
    // inject this - setter or constructor
    private Connection connection;

    public int getTweetCount(int tid) throws SQLException {
        int tweetCount = -1;
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            ps = this.connection.prepareStatement(SELECT_TWEETS);
            ps.setInt(1, tid);
            rs = ps.executeQuery();
            while (rs.hasNext()) {
                tweetCount = rs.getInt("TC");
            }
        } finally {
            DatabaseUtils.close(rs);
            DatabaseUtils.close(ps);
        }
        return tweetCount;
    }
}

Answer 2

尝试PreparedStatement：

String sql = "select count(tid) from retweets where tid = 35";
PreparedStatement stmt = m_con.prepareStatement(sql);

Answer 3

**try {
            ps = this.connection.prepareStatement(SELECT_TWEETS);
            ps.setInt(1, tid);
            rs = ps.executeQuery();
            while (rs.hasNext()) {
                tweetCount = rs.getInt("TC");
            }
        } finally {
            DatabaseUtils.close(rs);`enter code here`**

我认为这里不需要使用while循环，因为我们只得到单个结果。如果我错了，请告诉我。

JDBC ResultSet聚合函数

3 个答案: