send the name of database in a variable - sql-server

Question

I have this query in a SP:
select #QUERY =
COALESCE(#QUERY+' UNION ALL ','') +
'SELECT ''' +
c.name + ''' AS [ID], CAST([' + c.name + '] AS VarChar(MAX)) as [Value],''' +
t.name + ''' AS [Type] FROM ' + #dynDb + '.dbo.' +
#TableName +
' WHERE actorId = '+ CAST(#ActorId as nvarchar(max)) +
' and languageId = ''' + CAST(#LanguageId AS NVARCHAR(max))+ ''''
FROM cmssystemprocess.dbo.syscolumns c
INNER JOIN cmssystemprocess.dbo.SYSTYPES T ON T.XTYPE = C.XTYPE AND T.NAME != 'SYSNAME'
INNER JOIN cmssystemprocess.dbo.sysobjects o on c.id = o.id and o.xtype = 'u'
WHERE
o.name = #TableName and
(c.name like 'C%' or c.name like 'id') and
c.name not like 'created%'
ORDER BY c.colid
I want to get the name of database here in a variable (lets say #dyndb) and put it in the query instead of cmssystemprocess.dbo..

Answer 1

The only way to parameterize the database you're querying is with dynamic SQL, which means your proc will have to create dynamic SQL within dynamic SQL. That can get ugly very quickly.
Some tips:
use sp_executesql and pass in all of your variables (#dynDb, #ActorId, etc), as well as #QUERY as an output variable, via the #params parameter.
construct the dynamic SQL string using backticks (`) rather than doubling up on the single quotes. Then, once the string is built, replace(#QUERY,'`','''').
when building dynamic SQL within dynamic SQL, use a different single characters in place of single quotes in each "level" of the SQL.
Using the SQL query in your question, I would
Change all instances of doubled-up single quotes to a single backtick, for readability.
Wrap the select statement with replace(...,'`','''') to restore the doubled-up quotes
Take the result of that and replace all single quotes with another seldom-used-but-reminiscent-of-a-quote character such as ^
replace the instances of cmssystemprocess with a placeholder such as #DB#
Wrap the entire thing with two replace() functions to embed the DB name and change all the ^ back to doubled-up single quotes
Here's a mock-up:
-- Mimic stored proc parms
declare #QUERY nvarchar(max)
,#SpDbParam nvarchar(128) = 'cmssystemprocess'
,#dynDb nvarchar(128)
,#ActorId nvarchar(max)
,#LanguageId nvarchar(max)
,#TableName nvarchar(128)
; -- end stored proc parms mockup
-- Actual code for your proc follows
declare #CMD nvarchar(max)
,#Params nvarchar(max) = '
#dynDb nvarchar(128)
,#ActorId nvarchar(max)
,#LanguageId nvarchar(max)
,#TableName nvarchar(128)
,#QUERY nvarchar(max) output';
set #CMD = replace(replace('
select #QUERY = replace(
COALESCE(#QUERY+^ UNION ALL ^,^^) +
^SELECT `^ +
c.name + ^` AS [ID], CAST([^ + c.name + ^] AS VarChar(MAX)) as [Value],`^ +
t.name + ^` AS [Type] FROM ^ + #dynDb + ^.dbo.^ +
#TableName +
^ WHERE actorId = ^+ CAST(#ActorId as nvarchar(max)) +
^ and languageId = `^ + CAST(#LanguageId AS NVARCHAR(max))+ ^`^
,^`^,^^^^)
FROM #DB#.dbo.syscolumns c
INNER JOIN #DB#.dbo.SYSTYPES T ON T.XTYPE = C.XTYPE AND T.NAME != ^SYSNAME^
INNER JOIN #DB#.dbo.sysobjects o on c.id = o.id and o.xtype = ^u^
WHERE
o.name = #TableName and
(c.name like ^C%^ or c.name like ^id^) and
c.name not like ^created%^
ORDER BY c.colid'
,'#DB#',quotename(#SpDbParam))
,'^' ,'''');
print #CMD; -- for debugging / showing output in this stackoverflow post
exec sp_executesql #CMD, #Params, #dynDb, #ActorId, #LanguageId, #TableName, #QUERY output;
select #QUERY = replace(
COALESCE(#QUERY+' UNION ALL ','') +
'SELECT `' +
c.name + '` AS [ID], CAST([' + c.name + '] AS VarChar(MAX)) as [Value],`' +
t.name + '` AS [Type] FROM ' + #dynDb + '.dbo.' +
#TableName +
' WHERE actorId = '+ CAST(#ActorId as nvarchar(max)) +
' and languageId = `' + CAST(#LanguageId AS NVARCHAR(max))+ '`'
,'`','''')
FROM [cmssystemprocess].dbo.syscolumns c
INNER JOIN [cmssystemprocess].dbo.SYSTYPES T ON T.XTYPE = C.XTYPE AND T.NAME != 'SYSNAME'
INNER JOIN [cmssystemprocess].dbo.sysobjects o on c.id = o.id and o.xtype = 'u'
WHERE
o.name = #TableName and
(c.name like 'C%' or c.name like 'id') and
c.name not like 'created%'
ORDER BY c.colid