SQL request over multiple databases and tables - sql

Question

I'm having quite the problem.
The process data from our machines is stored in multiple MS SQL Databases.
It is possible to mount and unmount data which is no longer used or which should be archived.
Thats the reason, why there are multiple databases.
In each Database there exists multiple tables with the data values for one or more measuring points.
Which a JOIN Query i can get the values from one table with the corresponding table and tagname:
> SELECT HA_DATA_yyyy.R_xxxxx.time HA_DATA_yyyy.R_xxxxx.value, HA_DATA_yyyy.tags.tagname FROM HA_DATA_yyyy.R_xxxxx
> INNER JOIN HA_DATA_yyyy.RI_xxxxx ON HA_DATA_yyyy.R_xxxxx.id = HA_DATA_yyyy.RI_xxxxx.id
> INNER JOIN HA_DATA_yyyy.tags on HA_DATA_yyyy.RI_xxxxx.tag_uuid = HA_DATA_yyyy.tags.tag_uuid
> WHERE (HA_DATA_yyyy.tags.tagname like 'tagname')
yyyy - represents a number for the database
xxxxx - represents a number which is unique on the database-server, but differents in each database.
But now I'm looking for a solution to get this for all R_xxxxx tables of a database and for all mounted databases.
Is there any way to do this without external software? Just with the right query request or user defined function / stored procedure?

Answer 1

maybe dynamic sql is an option.
as a starting point you could use the list of databases:
insert into #dblist (dbname)
select d.name from sys.databases d where d.name like 'HA_DATA_%'
then for each database gather the list of tables to read data from (you can go with cursors or other loop as you prefer):
declare #dbname varchar(128) = ''
declare #dynsql nvarchar(max)
create table #listoftables (name varchar(128), db varchar(128))
while exists(select top 1 dbname from #dblist where dbname > #dbname order by dbname)
begin
set #dynsql = 'insert into #listoftables(name,db) select name,''' + #db + ''' from '+ #db +'.sys.tables'
exec sp_executesql #statement = #dynsql
-- move on to next db
select top 1 #dbname = dbname from #dblist where dbname > #dbname order by dbname
end
now you have a table list to loop onto to build a dynamic query to read all the data at once.
beware of the many issues you may incur using dynamic sql; here and there you can find just the first 2 results gathered with some explanations on why you have to be careful using dynamic sql.

Answer 2

Please have a look at the answer of this Stackoverflow question:
Archiving large amounts of old data in SQL Server
I think it might be what you need.
Update:
You can query the mounted databases by using the SQL query:
select Name into #myDatabases -- get all mounted databases into #myDatabases
from sys.databases
where (has_dbaccess(name) > 0)
and name not in ('master', 'tempdb', 'model', 'msdb')
order by 1
select * from #myDatabases -- list all mounted databases
drop table #myDatabases -- don't forget to drop it at the end
This can be used to create a dynamic SQL statement, which you execute via the sp_executesql command:
EXECUTE sp_executesql #SQL_String, #Parameter_Definition, #Param1, ..., #ParamN
The parameter definition is a string containing the list of parameter names and their datatypes.
So you can build up your own query based on the database list above and then execute it.