在结果集 Java 中执行访问查询

Question

我正在尝试对 MDB 文件执行计数查询，并希望将其存储到局部变量中。

如果直接分配输出，我会收到错误“类型不匹配：无法从布尔值转换为 int”。

在尝试使用结果集时，我也遇到了类似的错误“类型不匹配：无法从布尔值转换为结果集”

代码如下：

String connectionString ="jdbc:odbc:Driver={Microsoft Access Driver (*.mdb)}; DBQ=C:\\test\\TestEJFolder\\BWC_Ejournal.mdb;";

        DriverManager.getConnection(connectionString, "", "");

        Connection conn = DriverManager.getConnection(connectionString, "", "");
        Connection conn1 = DriverManager.getConnection(connectionString, "", "");
        Connection conn2 = DriverManager.getConnection(connectionString, "", "");

        String sql = "SELECT * FROM Events";
        String dt = "SELECT TOP 1 [Date] FROM Events";
        String count = "SELECT COUNT(ID) FROM Events";

        Statement cmd = conn.createStatement(); 
        Statement cmd1 = conn1.createStatement();   
        Statement cmd2 = conn2.createStatement();   

        cmd.execute(sql);
        cmd1.execute(dt);
        cmd2.execute(count);

        ResultSet rc = cmd2.execute(count);

        int r_count = cmd2.execute(count);

需要帮助来解决这个问题。

Answer 1

错误的代码太多了。开始阅读JDBC tutorial。

试试这样的：

ResultSet rc = cmd2.execute(count);
int r_count = 0;
while (rc.next()) {
    r_count = rc.getInt(1);
}

您没有关闭任何 JDBC 资源，这只会让您伤心。

您应该外部化您的连接信息。

我会将您的更多内容封装到小的、孤立的方法中，而不是以这种方式将多个语句混杂在一起。

您应该考虑一个定义明确的、基于接口的持久层。

进行这些 SQL 查询private static final String。不需要他们是本地的。

我建议您尝试更多类似的方法。从一个界面开始：

package persistence;

import java.util.List;
import java.util.Map;

/**
 * EventDao
 * @author Michael
 * @link http://stackoverflow.com/questions/5016730/creating-a-dsn-less-connection-for-ms-access-within-java
 * @since 6/25/13 5:19 AM
 */
public interface EventDao {
    List<Map<String, Object>> findAllEvents();
}

然后写一个实现：

package persistence;

import java.sql.*;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * EventDaoImpl
 * @author Michael
 * @link http://stackoverflow.com/questions/17213307/execute-access-query-in-resultset-java/17213356?noredirect=1#comment25072519_17213356
 * @since 6/25/13 5:15 AM
 */
public class EventDaoImpl implements EventDao {

    private static final String DEFAULT_URL = "jdbc:odbc:Driver={Microsoft Access Driver (*.mdb)}; DBQ=C:\\test\\TestEJFolder\\BWC_Ejournal.mdb;";
    private static final String SQL_FIND_ALL_EVENTS = "SELECT * FROM Events";

    private Connection connection;

    public EventDaoImpl(Connection connection) {
        this.connection = connection;
    }

    @Override
    public List<Map<String, Object>> findAllEvents() {
        List<Map<String, Object>> results = new ArrayList<Map<String, Object>>();
        Statement st = null;
        ResultSet rs = null;
        try {
            st = this.connection.createStatement();
            rs = st.executeQuery(SQL_FIND_ALL_EVENTS);
            results = map(rs);
        } catch (SQLException e) {
            e.printStackTrace();  
            throw new RuntimeException(e);
        } finally {
            close(rs);
            close(st);
        }
        return results;
    }

    public static Connection createConnection(String driver, String url, String username, String password) throws ClassNotFoundException, SQLException {
        Class.forName(driver);
        if ((username == null) || (password == null) || (username.trim().length() == 0) || (password.trim().length() == 0)) {
            return DriverManager.getConnection(url);
        } else {
            return DriverManager.getConnection(url, username, password);
        }
    }

    public static void close(Connection connection) {
        try {
            if (connection != null) {
                connection.close();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }


    public static void close(Statement st) {
        try {
            if (st != null) {
                st.close();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public static void close(ResultSet rs) {
        try {
            if (rs != null) {
                rs.close();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public static void rollback(Connection connection) {
        try {
            if (connection != null) {
                connection.rollback();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public static List<Map<String, Object>> map(ResultSet rs) throws SQLException {
        List<Map<String, Object>> results = new ArrayList<Map<String, Object>>();
        try {
            if (rs != null) {
                ResultSetMetaData meta = rs.getMetaData();
                int numColumns = meta.getColumnCount();
                while (rs.next()) {
                    Map<String, Object> row = new HashMap<String, Object>();
                    for (int i = 1; i <= numColumns; ++i) {
                        String name = meta.getColumnName(i);
                        Object value = rs.getObject(i);
                        row.put(name, value);
                    }
                    results.add(row);
                }
            }
        } finally {
            close(rs);
        }
        return results;
    }

    public static List<Map<String, Object>> query(Connection connection, String sql, List<Object> parameters) throws SQLException {
        List<Map<String, Object>> results = null;
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            ps = connection.prepareStatement(sql);

            int i = 0;
            for (Object parameter : parameters) {
                ps.setObject(++i, parameter);
            }
            rs = ps.executeQuery();
            results = map(rs);
        } finally {
            close(rs);
            close(ps);
        }
        return results;
    }
}