Question

我使用JavaDB / Derby创建了一个内存数据库表。我可以INSERT数据进入表格。但是，当我尝试从表中SELECT数据时，它不起作用。

我创建表：

DECLARE GLOBAL TEMPORARY TABLE SESSION.memtable (id int, name varchar(10))
NOT LOGGED

插入数据：

INSERT INTO SESSION.memtable (id, name) VALUES (?,?)

并为1返回rows affected。我选择数据：

SELECT name FROM SESSION.memtable WHERE id = ?

但它返回空 ResultSet（rs.next()为false）。

使用SELECT时我做错了什么？

我使用JDK7中包含的derby.jar。

这是我的代码：

public class DBTest {
    public static void main(String[] args) {
        final int userId = 4;
        final String sql = "DECLARE GLOBAL TEMPORARY TABLE SESSION.memtable "+
                            "(id int, name varchar(10)) NOT LOGGED";
        final String inSQL = "INSERT INTO SESSION.memtable (id, name) "+
                             "VALUES (?,?)";
        final String selSQL = "SELECT name FROM SESSION.memtable WHERE id = ?"
        final String connURL = "jdbc:derby:memory:memdatabase;create=true";
        try(Connection conn = DriverManager.getConnection(connURL);) {
            try (PreparedStatement ps = conn.prepareStatement(sql);) {
                ps.execute();
            }
            try (PreparedStatement ps = conn.prepareStatement(inSQL);) {
                ps.setInt(1, userId);
                ps.setString(2, "Jonas");
                int rows = ps.executeUpdate();
                System.out.println(rows + " rows inserted.");
            }
            try (PreparedStatement ps = conn.prepareStatement(selSQL);) {
                ps.setInt(1, userId);
                try (ResultSet rs = ps.executeQuery();) {
                    String name;
                    if(rs.next()) {
                        name = rs.getString("name");
                    } else {
                        name = null;
                    }
                    System.out.println("Name: " + name);
                }
            }
        } catch (SQLException e) {e.printStackTrace();}}}

更新

如果我将SELECT查询更改为：

SELECT t1.name AS name FROM SESSION.memtable t1 WHERE id = ?

我收到此错误消息：

java.sql.SQLException: Operation 'DROP TABLE' cannot be performed on object 'MEMTABLE(ID, NAME)' because there is an open ResultSet dependent on that object.
    at org.apache.derby.impl.jdbc.SQLExceptionFactory40.getSQLException(Unknown Source)
    at org.apache.derby.impl.jdbc.Util.generateCsSQLException(Unknown Source)
    at org.apache.derby.impl.jdbc.TransactionResourceImpl.wrapInSQLException(Unknown Source)
    at org.apache.derby.impl.jdbc.TransactionResourceImpl.handleException(Unknown Source)
    at org.apache.derby.impl.jdbc.EmbedConnection.handleException(Unknown Source)
    at org.apache.derby.impl.jdbc.EmbedConnection.commitIfAutoCommit(Unknown Source)
    at org.apache.derby.impl.jdbc.ConnectionChild.commitIfAutoCommit(Unknown Source)
    at org.apache.derby.impl.jdbc.EmbedStatement.resultSetClosing(Unknown Source)
    at org.apache.derby.impl.jdbc.EmbedResultSet.movePosition(Unknown Source)
    at org.apache.derby.impl.jdbc.EmbedResultSet.next(Unknown Source)
    at DBTest.main(DBTest.java:29)
Caused by: java.sql.SQLException: Operation 'DROP TABLE' cannot be performed on object 'MEMTABLE(ID, NAME)' because there is an open ResultSet dependent on that object.
    at org.apache.derby.impl.jdbc.SQLExceptionFactory.getSQLException(Unknown Source)
    at org.apache.derby.impl.jdbc.SQLExceptionFactory40.wrapArgsForTransportAcrossDRDA(Unknown Source)
    ... 11 more
Caused by: ERROR X0X95: Operation 'DROP TABLE' cannot be performed on object 'MEMTABLE(ID, NAME)' because there is an open ResultSet dependent on that object.
    at org.apache.derby.iapi.error.StandardException.newException(Unknown Source)
    at org.apache.derby.impl.sql.conn.GenericLanguageConnectionContext.verifyNoOpenResultSets(Unknown Source)
    at org.apache.derby.impl.sql.GenericPreparedStatement.prepareToInvalidate(Unknown Source)
    at org.apache.derby.impl.sql.depend.BasicDependencyManager.coreInvalidateFor(Unknown Source)
    at org.apache.derby.impl.sql.depend.BasicDependencyManager.invalidateFor(Unknown Source)
    at org.apache.derby.impl.sql.conn.GenericLanguageConnectionContext.tempTablesAndCommit(Unknown Source)
    at org.apache.derby.impl.sql.conn.GenericLanguageConnectionContext.doCommit(Unknown Source)
    at org.apache.derby.impl.sql.conn.GenericLanguageConnectionContext.userCommit(Unknown Source)
    at org.apache.derby.impl.jdbc.TransactionResourceImpl.commit(Unknown Source)
    ... 6 more

但Name:未打印，因此仍然无效。

Answer 1

在derby-user邮件列表中讨论了该问题，其中包含了on commit preserve rows的附加要求。

启用autocommit时，默认情况下会在提交时清除临时表，因此extra子句可确保在事务提交之后保持行可用。

Answer 2

这里使用两个概念，一个是带有In-Memors存储选项的Derby，另一个是临时表。如果需要In-Memors数据库，仍可以使用常规定义的表（“CREATE TABLE ...”）。这将使您更容易，因为您不必处理这些临时表的会话语义。

Answer 3

你应该真正关注Hibernate。它不仅可以映射您的类，还可以为您管理讨厌的JDBC内容，因此您可以专注于您想要完成的任务。它远非完美，但除此之外，它还为您提供了数据库可移植性。

我通过Spring使用hibernate，所以我的代码看起来像：

TempStorage temp = new TempStorage();
temp.setName("Bob");
getHibernateTemplate.insert(temp);
temp = getHibernateTemplate.find("SELECT name FROM TempStorage");

如何从内存中的Derby数据库表中选择数据？

3 个答案: