如何将数据从一个表复制到另一个表，然后使用 Java 删除第一个表中的数据？

Question

数据库中有两张表，一张是 Student 表，列有 roll_no(PK)、name、grade 和 DOB，另一张表 StudentLeft，列有 roll_no、name、grade 和 leave_date。

我想从学生表中删除用户输入卷号的学生的记录，并将卷号，姓名，年级和离开日期（删除记录并添加到表中的日期）添加到StudentLeft 表。

这是我的方法。

public static void main(String[] args) throws SQLException {
    Connection connection = null;
    PreparedStatement preparedStatement = null, preparedStatement1 = null, preparedStatement2 = null;
    ResultSet resultSet = null;
    String selectQuery = "", updateQuery = "", deleteQuery = "";

    try {
        Class.forName("oracle.jdbc.driver.OracleDriver");
        connection = dataSource.getConnection();
    }
    catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
    catch (SQLException e) {
        e.printStackTrace();
    }

    int rollNo = Integer.parseInt(args[0]);

    try {
        selectQuery = "SELECT name, grade FROM Student WHERE roll_no = ?";
        updateQuery = "INSERT INTO StudentLog values WHERE roll_no = ?, name = ?, standard = ?";
        deleteQuery = "DELETE Student WHERE roll_no = ?";
        
        connection.setAutoCommit(false);
        preparedStatement = connection.prepareStatement(selectQuery);
        preparedStatement.setInt(1, rollNo);
        resultSet = preparedStatement.executeQuery();
        preparedStatement1 = connection.prepareStatement(updateQuery);
        preparedStatement1.setInt(1, rollNo);
        
        while (resultSet.next()) {
            String name = resultSet.getString("name");
            String grade = resultSet.getString("grade");
            preparedStatement1.setString(2, name);
            preparedStatement1.setString(3, grade);
            preparedStatement1.addBatch();
        }
        preparedStatement1.executeBatch();
        preparedStatement2 = connection.prepareStatement(deleteQuery);
        preparedStatement.setInt(1, rollNo);
        connection.commit();
    }

    catch (SQLException e) {
        e.printStackTrace();
    }

    try {
        if (!preparedStatement.isClosed() || !preparedStatement1.isClosed() || !preparedStatement2.isClosed()) {
            preparedStatement.close();
            preparedStatement1.close();
            preparedStatement2.close();
        }

        if (!connection.isClosed())
            connection.close();
    }
    
    catch (SQLException e) {
        e.printStackTrace();
    }
}

这些是错误。

  java.sql.BatchUpdateException: ORA-00936: missing expression
    at oracle.jdbc.driver.OraclePreparedStatement.executeBatch(OraclePreparedStatement.java:10500)
    at oracle.jdbc.driver.OracleStatementWrapper.executeBatch(OracleStatementWrapper.java:230)    
    at Q3.main(Q3.java:48)
  Exception in thread "main" java.lang.NullPointerException
    at Q3.main(Q3.java:62)

我正在使用 oracle 11g express 数据库。

Answer 1

你写的代码可以简化不少：

  public static void main(String[] args) {
        try {
            Class.forName("oracle.jdbc.driver.OracleDriver");
        }
        catch (ClassNotFoundException e) {
            e.printStackTrace();
            return;
        }

        int rollNo = Integer.parseInt(args[0]);
        try (Connection connection = dataSource.getConnection()) {
            connection.setAutoCommit(false);

            String transferStatement = "INSERT INTO StudentLog (roll_no, name, standard, leaving_date) " +
                    "SELECT roll_no, name, standard, SYSDATE FROM Student WHERE roll_no = ?";
            try (PreparedStatement stmt = connection.prepareStatement(transferStatement)) {
                stmt.setInt(1, rollNo);
                stmt.executeUpdate();
            }

            String deleteStatement = "DELETE FROM Student WHERE roll_no = ?";
            try (PreparedStatement stmt = connection.prepareStatement(deleteStatement)) {
                stmt.setInt(1, rollNo);
                stmt.executeUpdate();
            }

            connection.commit();
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
    }

我使用了 try-with-resources 语句，它简化了连接和预处理语句的清理：当try (...) 块内的代码完成执行时，连接和语句将关闭。

将数据从Student 表传输到StudentLog 表可以使用INSERT INTO ... SELECT 语句一次性完成。该语句不返回任何结果集：没有什么可迭代的，我们只是执行它并插入行。

DELETE 语句类似：它也没有返回结果集。我已经将关键字 FROM 添加到其中，而不是其他任何东西：正如在另一个答案中指出的那样，FROM 是可选的。

我还将catch (SQLException e) 块移到了末尾：它将处理连接到数据库或执行任一准备好的语句时生成的所有 SQLExceptions。

我保留了尝试加载 Oracle 数据库驱动程序类的代码，但在 catch 块中添加了 return 语句：如果出现异常，则驱动程序不在类路径中并连接到数据库保证会失败，所以我们不妨停止。但是，对于最新版本的 Oracle 驱动程序，您不需要此检查。试验一下：看看代码在没有这个检查的情况下是否可以工作，如果可以，删除它。

Answer 2

你的查询不应该是

DELETE FROM Student WHERE roll_no = ?

而不是

DELETE Student WHERE roll_no = ?

Answer 3

您的 DELETE 代码使用了错误的预处理语句，缺少执行。

建议使用如下 try-with-resources 来自动关闭， 即使是退货或例外。 （它还负责变量范围。）

public static void main(String[] args) throws SQLException {
    int rollNo = Integer.parseInt(args[0]);

    // Better statements possible.
    final String selectQuery = "SELECT name, grade FROM Student WHERE roll_no = ?";
    final String updateQuery =
        "INSERT INTO StudentLog VALUES WHERE roll_no = ?, name = ?, standard = ?";
    final String deleteQuery = "DELETE FROM Student WHERE roll_no = ?";

    try { // Check whether you need this. It is for the old discovery mechanism.
        Class.forName("oracle.jdbc.driver.OracleDriver");
    }
    catch (ClassNotFoundException e) {
        throw new IllegalStateException("Database driver not provided", e);
    }

    try (Connection connection = dataSource.getConnection()) {
        connection.setAutoCommit(false);
        
        try (PreparedStatement preparedStatement =
                connection.prepareStatement(selectQuery)) {
            preparedStatement.setInt(1, rollNo);
            try (ResultSet resultSet = preparedStatement.executeQuery()) {
                try (PreparedStatement preparedStatement1 =
                        connection.prepareStatement(updateQuery)) {
                    preparedStatement1.setInt(1, rollNo);

                    while (resultSet.next()) {
                        String name = resultSet.getString("name");
                        String grade = resultSet.getString("grade");
                        preparedStatement1.setString(2, name);
                        preparedStatement1.setString(3, grade);
                        preparedStatement1.addBatch();
                    }
                    preparedStatement1.executeBatch();
                }
            }
        }
        try (PreparedStatement preparedStatement2 =
                connection.prepareStatement(deleteQuery)) {
            preparedStatement2.setInt(1, rollNo); // NOT preparedStatement
            preparedStatement2.executeUpdate();
        }
        connection.commit();
    }
}

然后一个应该SELECT+INSERT到数据库，使用一个语句（INSERT SELECT）。

StudentLog 的 SQL 对我来说有点难以理解，但一个不错的 INSERT 应该是：

INSERT INTO StudentLog VALUES(roll_no, name, standard)
SELECT roll_no, name, grade
FROM Student
WHERE roll_no = ?

消除了对数据库访问的 Java 嵌套。