调用两个数据库服务器的最佳方式 [英] best way to call two database servers

问题描述

我想一次调用不同数据库的两个表.数据库A的表包含用户ID，数据库B的表包含用户位置和用户ID，所以我想加入这两个表并获取user_id对应的位置.database A 和 B 位于两个不同的服务器中.那么我怎样才能加入这两个表.如果不可能加入这个，有没有什么有效的方法可以做到这一点.请帮忙.我正在尝试使用 java、mysql 来做到这一点.

i want to call two table of different database at once.table of database A contains user id and table of database B contains user location with the user id, so i want to join these two tables and get the location corresponding to user_id.database A and B are in two different servers. so how can i join these two tables. if it is not possible to join this, is there any efficient way to do this.please help.i'm trying to do this using java, mysql.

 ps = con.prepareStatement("SELECT user_id FROM A.users");
 rs = ps.executeQuery();
 while(rs.next())
    {
    //call select statement for database B to get the location for each user id             

    }

请提出一种有效的方法来做到这一点

please suggest an efficient way to do this

Id  User_id
===========
1   44
2   23

User_id     location
====================
44          india
23          us

推荐答案

假设 user_id 是一个 long.

PreparedStatement psUserLocation = conB.prepareStatement("SELECT location FROM B.users WHERE user_id = ?");
while(rs.next()) {
    //call select statement for database B to get the location for each user id
    long userId = rs.getLong(user_id);
    psUserLocation.setLong(1, userId)
    ResultSet userLocation = ps.executeQuery();
    // Do whatever with the location(s)
}

EDIT:针对所有用户的一个查询，而不是每个用户一个查询:

EDIT: one query for all users instead of one query per user:

private final static String QUERY = "SELECT user_id, location FROM B.users WHERE user_id IN (%a)";

StringBuilder userList = new StringBuilder();
while(rs.next()) {
    long userId = rs.getLong(user_id);
    userList.append(userId);
    if (!rs.isLast()) {
        userList.append(",");
    }
}

String usersLocationQuery = QUERY.replaceAll("%a", userList.toString());
PreparedStatement psUsersLocation = conB.prepareStatement(usersLocationQuery);
ResultSet usersLocation = psUsersLocation.executeQuery();
// Do whatever with the locations

请记住，这可能会失败/出错，因为大多数数据库对 SQL IN 子句可以包含的项目数有限制.此外，第二种方法可能允许对 %a 替换进行 SQL 注入.

Keep in mind this can fail/work wrong because most DB have a limit for how many items an SQL IN clause can include. Also this second method might allow an SQL injection on the %a replacement.

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