Mysql Cross Schema Queries Based on Row Data - sql

Question

So I'm working on a web application which our clients want their user data separated from other client's user data. Since our clients can be collaborative with each other, we can't just create a new instance of the application for each client, since we have to make sure user ids match up and are unique between clients.
Basically there's our "MasterApplication" database which has a "User" table which contains a UserId and the name of the database where to find the remaining user data. Each client has their own database user which only has access to the "MasterApplication" and their own client table.
Here's an example of how it looks.
CREATE DATABASE MasterDatabase
CREATE DATABASE Client1
CREATE DATABASE Client2
CREATE TABLE `MasterDatabase`.`Person` (
`PersonId` INT NOT NULL AUTO_INCREMENT,
`DatabaseName` Varchar(20) NOT NULL,
PRIMARY KEY (`PersonId`)
)
CREATE TABLE `Client1`.`Person` (
`PersonId` INT NOT NULL,
`FirstName` Varchar(20) NOT NULL,
PRIMARY KEY (`PersonId`)
)
CREATE TABLE `Client2`.`Person` (
`PersonId` INT NOT NULL,
`FirstName` Varchar(20) NOT NULL,
PRIMARY KEY (`PersonId`)
)
INSERT INTO MasterDatabase.Person VALUES (1, 'Client1');
INSERT INTO MasterDatabase.Person VALUES (2, 'Client2');
INSERT INTO Client1.Person VALUES (1, 'Matt');
INSERT INTO Client2.Person VALUES (2, 'Scott');
So Client1 would have a database user which has access to MasterDatabase and Client1 tables. Client2 would have a user with access to MasterDatabase and Client2.
I'm hoping there's some way that I can do a cross schema query easily using the data from the "DatabaseName" field in MasterDatabase, but I can't figure it out. I know for the most part I can store the client's database name in the application logic and just insert it into the query, but there will be a few spots which I'll need to join all clients into one query.
My attempt was to do
SELECT *, `DatabaseName` INTO #DB FROM MasterDatabase.Person
LEFT JOIN #DB.Person ON MasterDatabase.Person.PersonId = #DB.Person.PersonId
But that didn't work as I was hoping. Is there maybe someway I can do it with a procedure? I'm also open to other ideas of separating out the user data per client too if you have any.

Answer 1

I think you can do it in stored procedure. It will look something like
DELIMITER //
CREATE DEFINER='root'#'localhost' PROCEDURE GetData(IN user_id INT)
BEGIN
DECLARE db_name varchar(100);
SELECT DatabaseName INTO db_name FROM MasterDatabase.Person WHERE PersonId = user_id;
SET #t1 =CONCAT('SELECT * FROM ',db_name,'.Person' );
PREPARE stmt1 FROM #t1;
EXECUTE stmt1;
END//
UPDATE Sorry, I forgot DEALLOCATE PREPARE stmt1; ; it should be after EXECUTE stmt1;