最简单的方法，运行Count(*)查询，执行resultSet.next()指向第一行，然后执行resultSet.getString(1)获得计数。代码:

ResultSet rs = statement.executeQuery("Select Count(*) from your_db");
if(rs.next()) {
   int count = rs.getString(1).toInt()
}

为列命名。

String query = "SELECT COUNT(*) as count FROM

引用该列从ResultSet对象到int，并从那里做你的逻辑。

PreparedStatement statement = connection.prepareStatement(query);
statement.setString(1, item.getProductId());
ResultSet resultSet = statement.executeQuery();
while (resultSet.next()) {
    int count = resultSet.getInt("count");
    if (count >= 1) {
        System.out.println("Product ID already exists.");
    } else {
        System.out.println("New Product ID.");
    }
}

如果你有一个ResultSet类型的ResultSet。TYPE_FORWARD_ONLY您希望保持这种方式(而不是切换到ResultSet)。TYPE_SCROLL_INSENSITIVE或ResultSet。TYPE_SCROLL_INSENSITIVE，以便能够使用.last())。

我建议一种非常有效的破解方法，即在顶部添加第一个包含行数的伪/伪行。

例子

假设您的查询如下

select MYBOOL,MYINT,MYCHAR,MYSMALLINT,MYVARCHAR
from MYTABLE
where ...blahblah...

输出是这样的

true    65537 "Hey" -32768 "The quick brown fox"
false  123456 "Sup"    300 "The lazy dog"
false -123123 "Yo"       0 "Go ahead and jump"
false       3 "EVH"    456 "Might as well jump"
...
[1000 total rows]

简单地重构你的代码，就像这样:

Statement s=myConnection.createStatement(ResultSet.TYPE_FORWARD_ONLY,
                                         ResultSet.CONCUR_READ_ONLY);
String from_where="FROM myTable WHERE ...blahblah... ";
//h4x
ResultSet rs=s.executeQuery("select count(*)as RECORDCOUNT,"
                           +       "cast(null as boolean)as MYBOOL,"
                           +       "cast(null as int)as MYINT,"
                           +       "cast(null as char(1))as MYCHAR,"
                           +       "cast(null as smallint)as MYSMALLINT,"
                           +       "cast(null as varchar(1))as MYVARCHAR "
                           +from_where
                           +"UNION ALL "//the "ALL" part prevents internal re-sorting to prevent duplicates (and we do not want that)
                           +"select cast(null as int)as RECORDCOUNT,"
                           +       "MYBOOL,MYINT,MYCHAR,MYSMALLINT,MYVARCHAR "
                           +from_where);

您的查询输出将如下所示

1000 null     null null    null null
null true    65537 "Hey" -32768 "The quick brown fox"
null false  123456 "Sup"    300 "The lazy dog"
null false -123123 "Yo"       0 "Go ahead and jump"
null false       3 "EVH"    456 "Might as well jump"
...
[1001 total rows]

所以你必须

if(rs.next())
    System.out.println("Recordcount: "+rs.getInt("RECORDCOUNT"));//hack: first record contains the record count
while(rs.next())
    //do your stuff

theStatement=theConnection.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_READ_ONLY);

ResultSet theResult=theStatement.executeQuery(query); 

//Get the size of the data returned
theResult.last();     
int size = theResult.getRow() * theResult.getMetaData().getColumnCount();       
theResult.beforeFirst();

我也有同样的问题。以这种方式使用ResultSet.first()在执行后解决它:

if(rs.first()){
    // Do your job
} else {
    // No rows take some actions
}

文档(链接):

布尔第一次() 抛出SQLException异常 将光标移动到此ResultSet对象的第一行。 返回: 如果光标位于有效的 行;如果结果集中没有行，则为False 抛出: SQLException—如果发生数据库访问错误;在封闭的结果集或结果集类型为TYPE_FORWARD_ONLY时调用此方法 SQLFeatureNotSupportedException—如果JDBC驱动程序不支持 这个方法 自: 1．2

(速度考虑)

很多人在这里建议ResultSet.last()，但你需要打开连接作为一个ResultSet。TYPE_SCROLL_INSENSITIVE，对于Derby嵌入式数据库，它比ResultSet.TYPE_FORWARD_ONLY慢10倍。

根据我对嵌入式Derby和H2数据库的微型测试，在SELECT之前调用SELECT COUNT(*)要快得多。

下面是我的代码和基准测试的更详细信息