Creating view from multiple databases linked by columns with db name - sql-server

Question

Hi I need to create a view or stored procedure that combines data and returns a result set from 3 different databases on the same server using a column that holds a schema (db) name.
For Example on the first DB I have this table:
CREATE TABLE [dbo].[CloudUsers](
ID int IDENTITY(1,1) NOT NULL,
Username nvarchar(50) NULL,
MainDB nvarchar(100) NULL
) ON [PRIMARY]
Each CloudUser has a separate DB so next now I need to fetch the data from the User database using the MainDB name. The data I need is always 1 row cause I'm using aggregate functions / query.
So in the User MainDB let's say I have this table.
CREATE TABLE [dbo].[CLIENT](
ID int NOT NULL,
Name nvarchar(50) NULL,
ProjectDBName [nvarchar](100) NULL
CreationDate datetime NULL
) ON [PRIMARY]
And I query like:
select min(CreationDate) from MainDB.Client
The same Idea for the Client I need to fetch even more data from a 3rd database that points to the Client ProjectDBName. Again it's aggregate data:
select Count(id) as TotalTransactions from ProjectDBName.Journal
My final result should have records from all databases. It's readonly data that I need for statistics.
Final result set example:
CloudUsers.Username, MainDB->CreationDate, ProjectDBName->TotalTransaction
How can I achieve that ?

Answer 1

This is not easy - and without a schema and sample data, I can't give you a precise answer.
You need to iterate through each client, and use dynamic SQL to execute a the query against the mainDB and projectDB join. You can either do that in one gigantic "union" query, or by creating a temporary table and inserting the data into that temporary table, and then selecting from the temp table at the end of the query.

Answer 2

For you who are curious of how to solve this issue I have found my own solution using some cursors + dynamic and a simple table variable, enjoy.
ALTER PROCEDURE CloudAnalysis as
DECLARE #objcursor cursor
DECLARE #innercursor cursor
DECLARE #userid int
,#maindb nvarchar(100)
,#clientid int
,#name nvarchar(50)
,#projdb nvarchar(100)
,#stat nvarchar(50)
,#sql nvarchar(max)
,#vsql nvarchar(max)
,#rowcount int
DECLARE #result table(userid int,clientid int,maindb nvarchar(100),name nvarchar(50),projdb nvarchar(100),stat nvarchar(50))
SET #objcursor = CURSOR FORWARD_ONLY STATIC FOR SELECT c.id,c.maindb,u.client_id FROM dbo.ClientUsers c join dbo.UserClients u on c.id = u.user_id open #objcursor
FETCH NEXT FROM #objcursor INTO #userid,#maindb,#clientid
WHILE (##FETCH_STATUS=0)
BEGIN
IF (EXISTS (SELECT name
FROM master.dbo.sysdatabases
WHERE ('[' + name + ']' = #maindb
OR name = #maindb)))
BEGIN
set #sql = N'SELECT #name = c.name,#projdb=c.ProjectDBName FROM ' + #maindb + '.dbo.CLIENT c WHERE c.id = ' + cast(#clientid as nvarchar)
EXECUTE sp_executesql #sql, N'#name NVARCHAR(50) OUTPUT,#projdb NVARCHAR(100) OUTPUT',
#name = #name OUTPUT
,#projdb = #projdb OUTPUT
SELECT #rowcount = ##ROWCOUNT
IF #rowcount > 0
BEGIN
--print ' client: ' + cast(#clientid as nvarchar)+
--':' + #name + ' projdb: ' + #projdb
IF (EXISTS (SELECT name
FROM master.dbo.sysdatabases
WHERE ('[' + name + ']' = #projdb
OR name = #projdb)))
BEGIN
SET #sql = N'SELECT #stat = j.stat FROM ' + #projdb + '.dbo.JournalTransaction j'
EXECUTE sp_executesql #sql
,N'#stat NVARCHAR(50) OUTPUT'
,#stat = #stat OUTPUT
END
INSERT INTO #result (userid,clientid,maindb,name,projdb,stat)
VALUES (#userid,#clientid,#maindb,#name,#projdb,#stat)
END
END
FETCH NEXT FROM #objcursor INTO #userid,#maindb,#clientid
END
CLOSE #objcursor
DEALLOCATE #objcursor
SELECT * FROM #result