How to Get Data from Multiple Database Dynamically? - mysql

Question

I have multiple databases in MySQL from Diffrent Companies.
Ex.
1.Company1
2.Company2
3.Company3
4.Company4
In every database the table and column's structure are same but data is stored for different companies.
Now if i have to get a count of EmployeeID's sales for diffrent companies then i need to write the queries like below.
Select a.EmployeeID,Count(b.TransactionDate)
From Company1.Employee as a
Inner Join Company1.Sales as b
On a.EmployeeID=b.EmployeeID
Group By a.EmployeeID
Union
Select a.EmployeeID,Count(b.TransactionDate)
From Company2.Employee as a
Inner Join Company2.Sales as b
On a.EmployeeID=b.EmployeeID
Group By a.EmployeeID
Union
Select a.EmployeeID,Count(b.TransactionDate)
From Company3.Employee as a
Inner Join Company3.Sales as b
On a.EmployeeID=b.EmployeeID
Group By a.EmployeeID
Union
Select a.EmployeeID,Count(b.TransactionDate)
From Company4.Employee as a
Inner Join Company4.Sales as b
On a.EmployeeID=b.EmployeeID
Group By a.EmployeeID
Notice i am changing the database in "FROM" Clause and "INNER JOIN" with hard-coded value.
In future further database may be added and i don't want to change the code behind or i don't want add code with another "union".
Is there anything which we can do to do it dynamically. i mean if we can store database name in a table and query should automatically pick those database information from the table.

Answer 1

I'm always waiting for upvotes and acknowledgements ;)
With this databases (they are all the same of course, so i post only one of them.
use `company3`;
DROP TABLE IF EXISTS Employee;
CREATE TABLE Employee
(`EmployeeID` int, `LastName` varchar(40), `Firstname` varchar(40), `Age` int)
;
INSERT INTO Employee
(`EmployeeID`, `LastName`, `Firstname`, `Age`)
VALUES
(1, 'Hansen', 'Han', 30),
(2, 'Svendson', 'Sven', 23),
(3, 'Pettersen', 'Peter', 20)
;
DROP TABLE IF EXISTS Sales;
CREATE TABLE Sales
(`EmployeeID` int, `TransactionDate` datetime)
;
INSERT INTO Sales
(`EmployeeID`, `TransactionDate`)
VALUES
(1, '2015-12-20 10:01:00'),
(1, '2015-12-20 10:01:00'),
(2, '2015-12-20 10:01:00'),
(2, '2015-12-20 10:01:00'),
(2, '2015-12-20 10:01:00')
;
And this stored procedure
CREATE DEFINER=`root`#`localhost` PROCEDURE `GetSakesConut`()
BEGIN
DECLARE bDone INT;
DECLARE DBname TEXT;
DECLARE sqlstement LONGTEXT;
DECLARE n INT;
DECLARE curs CURSOR FOR SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA
WHERE SCHEMA_NAME LIKE 'company%';
DECLARE CONTINUE HANDLER FOR NOT FOUND SET bDone = 1;
OPEN curs;
SET bDone = 0;
SET n =0;
SET sqlstement = '';
SALESloop: LOOP
FETCH curs INTO DBname;
IF bDone = 1 THEN
LEAVE SALESloop;
END IF;
IF n>0 THEN
SET sqlstement = CONCAT(sqlstement,' UNION ');
END IF;
SET sqlstement = CONCAT(sqlstement,'Select "',DBname,'",a.EmployeeID,');
SET sqlstement = CONCAT(sqlstement,'Count(b.TransactionDate) ');
SET sqlstement = CONCAT(sqlstement,'From ',DBname,'.Employee as a ');
SET sqlstement = CONCAT(sqlstement,'Inner Join ',DBname,'.Sales as b ');
SET sqlstement = CONCAT(sqlstement,'On a.EmployeeID=b.EmployeeID ');
SET sqlstement = CONCAT(sqlstement,'Group By a.EmployeeID ');
SET n =n+1;
END LOOP SALESloop;
CLOSE curs;
SET #sqlstement = sqlstement;
PREPARE stmt FROM #sqlstement;
EXECUTE stmt;
END
For the explanation:
For the cursor curs i get all Database names that start with compan
In the loop i get one Dataase name after another and i build with it
your select statement with the correct database names.
And of course ou have to add union to all Select without the first
you get folloowing Result
company1 EmployeeID Count(b.TransactionDate)
company1 1 2
company1 2 3
company2 1 2
company2 2 3
company3 1 2
company3 2 3
Of course i had to adept the select statement because yours didn't work properly.