SQL - WHERE statement with text added to value from table - sql-server

Question

I need to copy data from an old database to a newer one.
Both of these databases have a user setup table with the primary key of "USER ID".
The problem is, in the old database the users didn't have the domain in the name, but in the new one they have.
Example:
Primary Key old DB: USER1
Primary Key new DB: DOMAIN\USER1
This prevents a standard WHERE clause to update the correct user because it can't find it due to the domain being added.
My code:
'FROM [' + #src_DB + '].dbo.[' + #src_table + '] as src '
'WHERE [' + #dest_DB + '].dbo.[' + #dest_table + '].[User ID] = ' + #domain_name + 'src.[User ID]'
printing the result:
WHERE [Destination_DB].dbo.[Destination_Table].[User ID] = DOMAIN\src.[User ID]
The problem is it doesn't add the DOMAIN to the value but rather to the statement...
How can I add the Domain to the actual value of src.[User ID]?

Answer 1

I think there's a dot missing, and you should use QUOTENAME
'WHERE ' + QUOTENAME(destination_table) + '.[User ID] = ' + QUOTENAME(#domain) + '.' + QUOTENAME(source_table) + '.[User ID]'
Whenever you create a SQL statement dynamically it's a good idea to print it out, copy it into a new query window and check for syntax errors...
UPDATE You: Yes, both databases are in the same server
An object can be (fully) specified with
ServerName.DatabaseName.Schema.ObjectName
A table's column would add one more .ColumnName
When both objects live on the same server you can let the first part away.
Objects of the same database let this part away.
Objects of the default schema might be called with the ObjectName alone.
But if you state a DatabaseName you must also state a SchemaName!
Use QUOTENAME() to add the brackets and add just the dots via string concatenation (or use CONCAT()-function).
UPDATE 2 Did I get this wrong completely?
After you comment I think I understand it now: You want to compare the values of both [USER ID] columns, but the new is DOMAIN\MyUserId while the older was just MyUserId.
You have two approaches
Add the Domain\ as string to the value of [User ID]
Use SUBSTRING([User ID],CHARINDEX('\',[UserID])+1,1000) to cut the newer value down to the naked value of [User ID]
For the first something like this
'WHERE [' + #dest_DB + '].dbo.[' + #dest_table + '].[User ID] = ''' + #domain_name + ''' + src.[User ID]'
The second is quite clumsy with dynamically created SQL...