java - 如何获取 java.sql.ResultSet 的大小？

Question

这不应该是一个非常简单的操作吗？但是，我看到既size()没有方法也没有length()方法。

score 289 · Accepted Answer

SELECT COUNT(*) FROM ...改为查询。

或者

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

在任何一种情况下，您都不必遍历整个数据。

score 100 · Accepted Answer

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
}

score 20 · Accepted Answer

好吧，如果您有 aResultSet类型ResultSet.TYPE_FORWARD_ONLY，您希望保持这种方式（而不是切换到 a ResultSet.TYPE_SCROLL_INSENSITIVEorResultSet.TYPE_SCROLL_INSENSITIVE以便能够使用.last()）。

我建议一个非常好的和高效的 hack，在顶部添加一个包含行数的第一个虚假/虚假行。

例子

假设您的查询如下

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

你的输出看起来像

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]

只需将您的代码重构为如下所示：

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);

您的查询输出现在将类似于

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]

所以你只需要

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

score 14 · Accepted Answer

14

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

于 2014-04-18T16:07:49.050 回答

score 12 · Accepted Answer

使用时出现异常rs.last()

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

：

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

这是由于默认情况下它是ResultSet.TYPE_FORWARD_ONLY，这意味着您只能使用rs.next()

解决方案是：

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

score 5 · Accepted Answer

[速度考虑]

这里有很多 ppl 建议ResultSet.last()，但为此您需要打开连接ResultSet.TYPE_SCROLL_INSENSITIVE，因为 Derby 嵌入式数据库的速度比ResultSet.TYPE_FORWARD_ONLY.

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

这是我的代码和基准测试的更详细信息

score 4 · Accepted Answer

ResultSet 大小的获取方式，无需使用 ArrayList 等

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

现在你会得到大小，如果你想打印结果集，在打印之前也使用下面的代码行，

rs.beforeFirst();

score 3 · Accepted Answer

这是一种进行行数计数的简单方法。

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

score 2 · Accepted Answer

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);

score 1 · Accepted Answer

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();

score 1 · Accepted Answer

我检查了ResultSet接口的运行时值，发现它几乎一直是一个ResultSetImpl。ResultSetImpl 有一个方法调用getUpdateCount()它返回您正在寻找的值。

此代码示例应该足够了：
ResultSet resultSet = executeQuery(sqlQuery);
double rowCount = ((ResultSetImpl)resultSet).getUpdateCount()

我意识到向下转换通常是一个不安全的过程，但这种方法还没有让我失望。

score 1 · Accepted Answer

今天，我使用了这个逻辑，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.

score 0 · Accepted Answer

我遇到了同样的问题。ResultSet.first()在执行解决它之后以这种方式使用：

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

文档（链接）：

boolean first()
    throws SQLException
将光标移动到该ResultSet对象的第一行。

回报：

true如果光标在有效行上；false如果结果集中没有行

抛出：

SQLException- 如果发生数据库访问错误；此方法在关闭的结果集上调用或结果集类型为TYPE_FORWARD_ONLY

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

自从：

1.2

score 0 · Accepted Answer

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) 来获取计数。代码：

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

score -1 · Accepted Answer

给列一个名字..

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

将该列从 ResultSet 对象引用到一个 int 并从那里执行您的逻辑..

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.");
    }
}

java - 如何获取 java.sql.ResultSet 的大小？

15 回答 15

Related

Reference