我如何得到一个java.sql.ResultSet的大小?

ResultSet rs = ps.executeQuery();
int rowcount = 0;
if (rs.last()) {
rowcount = rs.getRow();
rs.beforeFirst(); // not rs.first() because the rs.next() below will move on, missing the first element
}
while (rs.next()) {
// do your standard per row stuff
}

改为执行SELECT COUNT(*) FROM ...查询。

或

int size =0;
if (rs != null)
{
rs.last();    // moves cursor to the last row
size = rs.getRow(); // get row id
}

在这两种情况下，都不需要遍历整个数据。

这是一种简单的行计数方法。

ResultSet rs = job.getSearchedResult(stmt);
int rsCount = 0;


//but notice that you'll only get correct ResultSet size after end of the while loop
while(rs.next())
{
//do your other per row stuff
rsCount = rsCount + 1;
}//end while

我在使用rs.last()时得到了一个异常

if(rs.last()){
rowCount = rs.getRow();
rs.beforeFirst();
}

：

java.sql.SQLException: Invalid operation for forward only resultset

这是因为默认情况下它是ResultSet.TYPE_FORWARD_ONLY，这意味着你只能使用rs.next()

解是:

stmt=conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,
ResultSet.CONCUR_READ_ONLY);

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();

好吧，如果你有一个类型为ResultSet.TYPE_FORWARD_ONLY的ResultSet，你希望它保持这种方式(并且不切换到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

我检查了结果集接口的运行时值，发现它几乎一直都是ResultSetImpl。ResultSetImpl有一个名为getUpdateCount()的方法，它返回你正在寻找的值。

我知道向下转换通常是一个不安全的过程，但这个方法还没有让我失败。

String sql = "select count(*) from message";
ps =  cn.prepareStatement(sql);


rs = ps.executeQuery();
int rowCount = 0;
while(rs.next()) {
rowCount = Integer.parseInt(rs.getString("count(*)"));
System.out.println(Integer.parseInt(rs.getString("count(*)")));
}
System.out.println("Count : " + rowCount);

int i = 0;
while(rs.next()) {
i++;
}

获取ResultSet大小的方法，不需要使用数组列表等

int size =0;
if (rs != null)
{
rs.beforeFirst();
rs.last();
size = rs.getRow();
}

现在你会得到大小，如果你想打印ResultSet，在打印之前使用以下一行代码，

rs.beforeFirst();

我也有同样的问题。在执行之后以这种方式使用ResultSet.first():

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

文档(链接):

boolean first()
throws SQLException

将光标移动到ResultSet对象的第一行。

返回:

true如果光标在有效的 行;false如果结果集中没有行

抛出:

SQLException -如果数据库访问错误发生;此方法在封闭结果集或结果集类型为TYPE_FORWARD_ONLY时调用

SQLFeatureNotSupportedException -如果JDBC驱动程序不支持 此方法

自:

1.2

(速度考虑)

很多人在这里建议ResultSet.last()，但为此你需要以ResultSet.TYPE_SCROLL_INSENSITIVE的形式打开连接，对于Derby嵌入式数据库来说，这是慢的10倍。

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

这里是更详细的我的代码和我的基准测试

今天，我用了这个逻辑，为什么我不知道得到RS的计数。

int chkSize = 0;
if (rs.next()) {
do {  ..... blah blah
enter code here for each rs.
chkSize++;
} while (rs.next());
} else {
enter code here for rs size = 0
}
// good luck to u.

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.");
}
}

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()
}