Unclosed quotation mark after the character string ')' - OPENQUERY - sql-server

Question

I have the following query, executed with OPENQUERY :
DECLARE #DOMAIN NVARCHAR = N'XXX'
DECLARE #QUERY NVARCHAR(MAX)= '
SELECT * FROM OPENQUERY( [XX\XX],''
SELECT CONCAT(CONCAT([Firstname],''),[Lastname]) AS [Owner]
FROM [Employee] '')'
EXECUTE XX_executesql #QUERY
When I execute , I get this following error :
Msg 102, Level 15, State 1, Line 4 Incorrect syntax near ')'. Msg 105,
Level 15, State 1, Line 5 Unclosed quotation mark after the character
string ')'.

Answer 1

If you print the #Query value, you will see the error root. So if you run the following code:
DECLARE #DOMAIN NVARCHAR = N'XXX';
DECLARE #QUERY NVARCHAR(MAX)= '
SELECT * FROM OPENQUERY( [XX\XX],''
SELECT CONCAT(CONCAT([Firstname],''),[Lastname]) AS [Owner]
FROM [XXX].[dbo].[Employee] '')'
PRINT #QUERY
You will get the following result:
SELECT * FROM OPENQUERY( [XX\XX],'
SELECT CONCAT(CONCAT([Firstname],'),[Lastname]) AS [Owner]
FROM [XXX].[dbo].[Employee] ')
Now it is clear why SQL Server returns the
Unclosed quotation mark after..
To solve it you need to keep in mind that, in order to have a single quote mark in the output in a string variable, you need to put two single quote mark.
Now you need to rewrite it as below:
DECLARE #DOMAIN NVARCHAR = N'XXX';
DECLARE #QUERY NVARCHAR(MAX)= '
SELECT * FROM OPENQUERY( [XX\XX],''
SELECT CONCAT(CONCAT([Firstname],''''),[Lastname]) AS [Owner]
FROM [XXX].[dbo].[Employee] '')'
PRINT #QUERY
aforementioned query will produce:
SELECT * FROM OPENQUERY( [XX\XX],'
SELECT CONCAT(CONCAT([Firstname],''),[Lastname]) AS [Owner]
FROM [XXX].[dbo].[Employee] ')
You can now simply replace Print with EXECUTE command and execute it!

Answer 2

You are not escaping your string quotes.
If you want to include an ' inside an string, you have to write it twice ''
So, for example, CONCAT([Firstname],'') would have to be CONCAT([Firstname],'''')
See How do I escape a single quote in SQL Server?
PS: And as #TT. has commented, in this case you will probably need to re-escape your quotes inside the openquery scope, because the openquery call will escape them again.
So CONCAT([Firstname],'') would in fact have to be CONCAT([Firstname],'''''''')