如何获得java.sql.ResultSet的大小？

Question

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

Answer 1

请SELECT COUNT(*) FROM ...改为查询.

要么

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.

Answer 2

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
}

Answer 3

好吧,如果你有一个ResultSet类型,ResultSet.TYPE_FORWARD_ONLY你想保持这种方式(而不是切换到ResultSet.TYPE_SCROLL_INSENSITIVE或ResultSet.TYPE_SCROLL_INSENSITIVE为了能够使用.last()).

我建议一个非常好的和有效的黑客,你在顶部添加第一个伪行/假行包含行数.

例

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]

只需将代码重构为以下内容:

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是这样的

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]

所以你必须这样做

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

Answer 4

使用时我遇到了异常 rs.last()

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

:

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

它是默认的ResultSet.TYPE_FORWARD_ONLY,这意味着你只能使用rs.next()

解决方案是:

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

Answer 5

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

Answer 6

【速度考虑】

这里的脂肪酶很多建议ResultSet.last()，但对于您需要打开的连接的ResultSet.TYPE_SCROLL_INSENSITIVE这对德比嵌入式数据库是高达10倍速度较慢比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 之前调用要快得多。

这是我的代码和基准测试的更详细信息