T-SQL context issue - sql-server

Question

I have created a script to compare the tables of two databases to find differences. I want to be able to manually set two variables (one for each database) and have the 'USE' statement use the variable value for one of the databases but it changes context and does work correctly.
This is what I am trying to use to populating a variable (#Use) with the USE code to execute
--set #Use = 'USE ' + #NewProdDB + ';SELECT name FROM sys.tables'
--EXEC (#Use)
This is the entire query:
--========================================================================================================
-- Used to find table values in the test upgrade system database not found the newly upgraded system
-- database. Make sure change the system database names before running the query.
-- Also, select the upgraded database to run against
--========================================================================================================
DECLARE #NewProdDB VARCHAR(100)
DECLARE #TestUpgradeDB VARCHAR(100)
DECLARE #Columns VARCHAR (MAX)
DECLARE #Query VARCHAR(MAX)
DECLARE #ResultsTest VARCHAR(MAX)
DECLARE #SetResultsCursor NVARCHAR(MAX)
DECLARE #Fetcher NVARCHAR(MAX)
DECLARE #Use NVARCHAR(50)
DECLARE #ListOfTables VARCHAR(100)
DECLARE #WTF NVARCHAR(max)
DECLARE #rescanCode nvarchar(max)
/* Enter Upgraded system DB here */
SET #NewProdDB = 'zSBSSYS'
/* Enter Test Upgrade system DB here */
SET #TestUpgradeDB = 'TESTzSBSSYS'
--set #Use = 'USE ' + #NewProdDB + ';SELECT name FROM sys.tables'
--EXEC (#Use)
SET NOCOUNT ON
set #rescanCode = 'DECLARE recscan CURSOR FOR
SELECT TABLE_NAME FROM ' + #TestUpgradeDB + '.INFORMATION_SCHEMA.TABLES
WHERE TABLE_NAME NOT LIKE ''Mbf%'' AND TABLE_TYPE = ''BASE TABLE''
ORDER BY TABLE_NAME'
exec (#rescanCode)
OPEN recscan
FETCH NEXT FROM recscan INTO #ListOfTables
WHILE ##fetch_status = 0
BEGIN
BEGIN TRY
/* START Table column query */
Declare #Table varchar(60)
set #Table = #ListOfTables
set #table = (SELECT top 1 name FROM sys.tables where name = #Table)
DECLARE
#sql1 nvarchar(max) = ''
,#INTOColumns NVARCHAR(MAX)=''
,#Where nvarchar(max) = ''
,#ColumnID Int
,#Column Varchar(8000)
,#Type Varchar(8000)
,#Length Int
,#del1 varchar(2) = ','
,#CRLF bit = 1
,#aliasfield varchar(5) = '[a].'
DECLARE CSR_Attributes CURSOR FOR
SELECT
[ColumnID] = Col.column_id,
[Column] = Col.Name,
[Type] = [types].name,
[Length] = Col.max_length
FROM sys.objects Obj, sys.columns Col, sys.types [types]
WHERE Obj.OBJECT_ID = Col.OBJECT_ID AND Obj.TYPE = 'U'
AND Col.system_type_id = [types].system_type_id
AND Obj.name = #table
AND Col.name <> 'tstamp'
ORDER BY Obj.name, Col.column_id, Col.name
OPEN CSR_Attributes
FETCH NEXT FROM CSR_Attributes INTO #ColumnID, #Column, #Type, #Length
WHILE ##FETCH_STATUS = 0
BEGIN
set #sql1 += #column + #del1
set #Where += 'b.' + #column + '=a.' + #column + ' and '
set #INTOColumns += '#INTOcol,'
FETCH NEXT FROM CSR_Attributes INTO #ColumnID, #Column, #Type, #Length
END
CLOSE CSR_Attributes
DEALLOCATE CSR_Attributes
SET #Columns = SUBSTRING(#sql1,1,len(#sql1)-1) -- get rid of last comma
SET #Where = SUBSTRING(#Where,1,len(#Where)-4) -- get rid of last 'and'
SET #INTOColumns = SUBSTRING(#INTOColumns,1,len(#INTOColumns)-1) -- get rid of last comma
/* END Table column query */
/* Create SELECT statement here */
SET #ResultsTest='SELECT TOP 1 ' + #Columns + '
FROM ' + #TestUpgradeDB + '..' + #Table + ' A
WHERE NOT EXISTS
(SELECT ' + #Columns + '
FROM ' + #NewProdDB + '..' + #Table + ' B WHERE ' + #Where + ')'
SET #SetResultsCursor = 'DECLARE DataReturned CURSOR FOR ' + #ResultsTest
exec (#SetResultsCursor)
OPEN DataReturned
SET #Fetcher = 'DECLARE #INTOcol NVARCHAR(Max)
FETCH NEXT FROM DataReturned INTO ' + #INTOColumns
exec (#Fetcher)
if ##FETCH_STATUS = 0
begin
CLOSE DataReturned
DEALLOCATE DataReturned
SET #Query='SELECT ' + #Columns + '
FROM ' + #TestUpgradeDB + '..' + #Table + ' A WHERE NOT EXISTS
(SELECT ' + #Columns + ' FROM ' + #NewProdDB + '..' + #Table + ' B WHERE ' + #Where + ')'
select #Table
exec (#Query)
end
else
begin
CLOSE DataReturned
DEALLOCATE DataReturned
end
end try
begin catch
--SELECT ERROR_MESSAGE() AS ErrorMessage;
end catch
FETCH NEXT FROM recscan INTO #ListOfTables
end
CLOSE recscan
DEALLOCATE recscan
SET NOCOUNT OFF

Answer 1

You can't do this:
DECLARE #DB varchar(10) = 'beep';
DECLARE #USE varchar(50) = 'USE '+ #DB;
EXEC sp_sqlexec #USE;
SELECT blah FROM bloop
Database context is only used during the #USE sql, then reverts
You can do this:
DECLARE #DB varchar(10) = 'beep';
DECLARE #SQL varchar(50) = 'USE '+#DB+'; SELECT blah FROM bloop;'
EXEC sp_sqlexec #USE;
That aside, look into SQL Server Data Tools (SSDT)
It does exactly what it seems you are trying to do, schema and/or data compare between databases, and it's very easy to use.