如何在一个查询中从多个数据库中选择记录?

I want to use multiple database because I have 1 database which contain 40 tables and it doesnt seem good.Thats why I want to create another database but problem is I am not able to join this databases at one query for example I have 'core' database and it contains users table which has user_id column second database is post database and I need get user_id from users table which is in 'core' database like below

SELECT post_message from posts where 'core'.user_id=123 

is that possible?or should I stick with one database?I have also foreign key problem which is related with database relation.

If it is SQL SERVER Product, You can call the Object residing on another DB like below

SELECT * FROM [DB_NAME].[Schema_Name].[Table_NAME]

For Example:

SELECT * FROM DB1.dbo.Employee

For MySQL : Please go through the below ans Select columns across different databases