如何确定获取的 Java ResultSet 是否为空？
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How to find out if a Java ResultSet obtained is empty?
提问by Bruce
Class.forName("org.sqlite.JDBC");
Connection conn =
DriverManager.getConnection("jdbc:sqlite:userdata.db");
Statement stat = conn.createStatement();
ResultSet rs = stat.executeQuery("SELECT * from table WHERE is_query_processed = 0;");
int rowcount = rs.getRow();
System.out.println("Row count = "+rowcount); // output 1
rs.first(); // This statement generates an exception
Why is it so?
为什么会这样？
采纳答案by Colin Gislason
The pattern I normally use is as follows:
我通常使用的模式如下：
boolean empty = true;
while( rs.next() ) {
// ResultSet processing here
empty = false;
}
if( empty ) {
// Empty result set
}
回答by karim79
You can do this too:
你也可以这样做：
rs.last();
int numberOfRows = rs.getRow();
if(numberOfRows) {
rs.beforeFirst();
while(rs.next()) {
...
}
}
回答by Inv3r53
while (results.next())
is used to iterate over a result set.so results.next() will return false if its empty.
用于迭代结果集。因此，如果结果为空，则 results.next() 将返回 false。
回答by jasonmp85
Here's a simple method to do it:
这是一个简单的方法：
public static boolean isResultSetEmpty(ResultSet resultSet) {
return !resultSet.first();
}
Caveats
注意事项
This moves the cursor to the beginning. But if you just want to test whether it's empty, you probably haven't done anything with it yet anyways.
这会将光标移动到开头。但是如果你只是想测试它是否为空，你可能还没有对它做任何事情。
Alternatively
或者
Use the first()method immediately, before doing any processing.
ResultSet rs = stat.executeQuery("SELECT * from table WHERE is_query_processed = 0;");
first()在进行任何处理之前立即使用该方法。ResultSet rs = stat.executeQuery("SELECT * from table WHERE is_query_processed = 0;");
if(rs.first()) {
// there's stuff to do
} else {
// rs was empty
}
References
参考
回答by stacker
Why is execution not entering the while loop?
为什么执行不进入while循环？
If your ResultSet is empty the rs.next()method returns false and the body of the while loop isn't entered regardless to the rownumber (not count) rs.getRow()returns. Colins example works.
如果您的 ResultSet 为空，则该rs.next()方法返回 false 并且无论行号（不计数）rs.getRow()返回如何，都不会输入 while 循环的主体。柯林斯的例子有效。
回答by BalusC
Shifting the cursor forth and back to determine the amount of rows is not the normal JDBC practice. The normal JDBC practice is to map the ResultSetto a Listof value objects each representing a table row entity and then just use the Listmethods to determine if there are any rows.
来回移动光标以确定行数不是正常的 JDBC 实践。通常的 JDBC 实践是将 映射ResultSet到List每个代表一个表行实体的值对象，然后只使用这些List方法来确定是否有任何行。
For example:
例如：
List<User> users = userDAO.list();
if (users.isEmpty()) {
// It is empty!
if (users.size() == 1) {
// It has only one row!
} else {
// It has more than one row!
}
where the list()method look like as follows:
该list()方法如下所示：
public List<User> list() throws SQLException {
Connection connection = null;
Statement statement = null;
ResultSet resultSet = null;
List<User> users = new ArrayList<User>();
try {
connection = database.getConnection();
statement = connection.createStatement();
resultSet = statement.executeQuery(SQL_LIST);
while (resultSet.next()) {
User user = new User();
user.setId(resultSet.getLong("id"));
user.setName(resultSet.getString("name"));
// ...
users.add(user);
}
} finally {
if (resultSet != null) try { resultSet.close(); } catch (SQLException logOrIgnore) {}
if (statement != null) try { statement.close(); } catch (SQLException logOrIgnore) {}
if (connection != null) try { connection.close(); } catch (SQLException logOrIgnore) {}
}
return users;
}
Also see this answerfor other JDBC examples.
另请参阅此答案以了解其他 JDBC 示例。
回答by santosh kumar t
CLOSE_CURSORS_AT_COMMIT
CLOSE_CURSORS_AT_COMMIT
public static final int CLOSE_CURSORS_AT_COMMIT
公共静态最终 int CLOSE_CURSORS_AT_COMMIT
The constant indicating that ResultSet objects should be closed when the method Connection.commit is called.
回答by joseluisbz
Try with this:
试试这个：
ResultSet MyResult = null;
MyResult = Conexion.createStatement().executeQuery("Your Query Here!!!");
MyResult.last();
int NumResut = MyResult.getRow();MyResult.beforeFirst();
//Follow with your other operations....
This manner you'll be able work normally.
这样你就可以正常工作了。
回答by Chamod
May be you can convert your resultset object into String object and check whether is it empty or not.
也许您可以将结果集对象转换为 String 对象并检查它是否为空。
`if(resultset.toString().isEmpty()){
// containg null result
}
else{
//This conains the result you want
}`
回答by Ullauri
This checks if it's empty or not while not skipping the first record
这会在不跳过第一条记录的情况下检查它是否为空
if (rs.first()) {
do {
// ResultSet is not empty, Iterate over it
} while (rs.next());
} else {
// ResultSet is empty
}
