如果您的表属于同一服务器(不考虑数据库(相同或不同))，则可以相互连接. 但是，如果它们属于不同的服务器，则MySql会将其支持为 FEDERATED Engine .您可以查看此问题以了解更多详细信息.

If your tables are of same server (irrespective of databases (same or differ)) then it is possible to join with eachother. But if they belong different servers then it supports by the MySql as FEDERATED Engine . You can have look to this question for more details.