这不应该是一个相当简单的操作吗?然而,我发现没有size()
或length()
方法。
这不应该是一个相当简单的操作吗?然而,我发现没有size()
或length()
方法。
建议使用 SELECT COUNT(*) FROM ...
查询。
或者
int size =0;
if (rs != null)
{
rs.last(); // moves cursor to the last row
size = rs.getRow(); // get row id
}
无论哪种情况,您都不需要循环整个数据。
ResultSet#last()
方法并不适用于所有类型的 ResultSet
对象,你需要确保使用的是 ResultSet.TYPE_SCROLL_INSENSITIVE
或者 ResultSet.TYPE_SCROLL_SENSITIVE
中的一种。 - Marius IonResultSet 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
}
getRow()
适用于TYPE_FORWARD_ONLY
结果集,而beforeFirst()
对这些结果集会抛出错误。那么这个答案不是有问题吗? - CodePro_NotYetps=conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_READ_ONLY);
- BullyWiiPlaza如果您有一个类型为ResultSet.TYPE_FORWARD_ONLY
的ResultSet
,则希望保持它的状态(而不是切换到ResultSet.TYPE_SCROLL_INSENSITIVE
或ResultSet.TYPE_SCROLL_SENSITIVE
以便使用.last()
)。
我建议使用一种非常好的、高效的技巧,在顶部添加一个第一个虚假/伪行,其中包含行数。
示例
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]
只需将您的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类似于以下内容:
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
ResultSet.TYPE_FORWARD_ONLY
)时,这非常有用。 - Unai Viviint i = 0;
while(rs.next()) {
i++;
}
在使用rs.last()
时,我遇到了一个异常。
if(rs.last()){
rowCount = rs.getRow();
rs.beforeFirst();
}
java.sql.SQLException: Invalid operation for forward only resultset
这是因为默认是 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);
ResultSet.TYPE_FORWARD_ONLY
切换到 ResultSet.TYPE_SCROLL_INSENSITIVE
通常会导致巨大的性能损失。 - Unai ViviSELECT COUNT(*) FROM default_tbl
之前使用了SELECT COUNT(*) FROM default_tbl
时,总共只需要不到1.5秒钟。我在嵌入式Derby数据库10.11.1.1上进行了测试。 - Vit Bernatik[速度考虑]
这里有很多人建议使用ResultSet.last()
,但为此您需要将连接作为ResultSet.TYPE_SCROLL_INSENSITIVE
打开,对于Derby嵌入式数据库来说,这比ResultSet.TYPE_FORWARD_ONLY
慢高达10倍。
根据我的微型测试结果,在嵌入式Derby和H2数据库中,在SELECT之前调用SELECT COUNT(*)
明显更快。
int size =0;
if (rs != null)
{
rs.beforeFirst();
rs.last();
size = rs.getRow();
}
现在您将获得大小,如果您想要打印ResultSet,在打印之前也请使用以下代码行:
rs.beforeFirst();
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
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 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.