连接到同一DBMS上的“外部”数据库

令我惊讶的是，JDBC使我可以从具有相同特权的同一个DBMS上的“外部”数据库中的表中进行透明选择，而无需显式连接到外部数据库。这是MySQL应该采用的方式，还是只是JDBC的怪癖？

详细信息：
我在DBMS上创建了两个数据库：stkovrflo_1和stkovrflo_2。我从MySQL World数据库填充了这些数据库中的表。

CREATE TABLE stkovrflo_1.Country
SELECT name, region FROM world.Country;

CREATE TABLE stkovrflo_2.City
SELECT world.City.name, world.Country.name AS country
FROM world.City INNER JOIN world.Country ON world.City.CountryCode = world.Country.code;

在JDBC中，我可以stkovrflo_2.City通过与stkovrflo_1数据库的连接选择表的条目。我对这两个数据库都有SELECT访问权限。

这是我的JDBC代码：

import java.sql.*;

public class JDBCExample {
    static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";  
    static final String DB_URL = "jdbc:mysql://localhost/stkovrflo_1";

    public static void main(String[] args) throws Exception{
        
        String uid = args[0];
        String pswd = args[1];
        
        if(pswd.toUpperCase().equals("NULL"))
            pswd = null;

        Connection conn = null;
        conn = DriverManager.getConnection(DB_URL,uid,pswd);
        
        processTableSameDB(conn);
        System.out.println("\n\n");
        processTableDifferentDB(conn);
        
        if(conn != null)
            conn.close();
    }

    protected static void processTableSameDB(Connection conn) throws Exception {
        Statement stmt = null;
        String tableName = "Country";

        try{
            Class.forName("com.mysql.jdbc.Driver");

            System.out.println("Retrieving from table in same database...");

            stmt = conn.createStatement();
            String sql;
            sql = "SELECT * FROM " + tableName + " LIMIT 10";
            ResultSet rs = stmt.executeQuery(sql);

            while(rs.next()){
                String name = rs.getString("name");
                String region = rs.getString("region");

                System.out.print("Name: " + name);
                System.out.println(", Region: " + region);
            }
            rs.close();
        }
        catch(SQLException se){
            se.printStackTrace();
        }
        finally{
            if(stmt!=null)
                stmt.close();
        }
    }

    protected static void processTableDifferentDB(Connection conn) throws Exception {
        Statement stmt = null;
        String tableName = "stkovrflo_2.City";

        try{
            Class.forName("com.mysql.jdbc.Driver");

            System.out.println("Retrieving from table in different database...");

            stmt = conn.createStatement();
            String sql;
            sql = "SELECT * FROM " + tableName + " LIMIT 10";
            ResultSet rs = stmt.executeQuery(sql);

            while(rs.next()){
                String name = rs.getString("name");
                String country = rs.getString("Country");

                System.out.print("Name: " + name);
                System.out.println(", Country: " + country);
            }
            rs.close();
        }
        catch(SQLException se){
            se.printStackTrace();
        }
        finally{
            if(stmt!=null)
                stmt.close();
        }
    }
}  

JDBC输出如下：

Retrieving from table in same database...
Name: Aruba, Region: Caribbean
Name: Afghanistan, Region: Southern and Central Asia
Name: Angola, Region: Central Africa
Name: Anguilla, Region: Caribbean
Name: Albania, Region: Southern Europe
Name: Andorra, Region: Southern Europe
Name: Netherlands Antilles, Region: Caribbean
Name: United Arab Emirates, Region: Middle East
Name: Argentina, Region: South America
Name: Armenia, Region: Middle East



Retrieving from table in different database...
Name: Oranjestad, Country: Aruba
Name: Kabul, Country: Afghanistan
Name: Qandahar, Country: Afghanistan
Name: Herat, Country: Afghanistan
Name: Mazar-e-Sharif, Country: Afghanistan
Name: Luanda, Country: Angola
Name: Huambo, Country: Angola
Name: Lobito, Country: Angola
Name: Benguela, Country: Angola
Name: Namibe, Country: Angola