Create database user conditionally syntax error - database

Question

I'm using the following to create a user if it doesn't already exist in the database:
use DBExample
GO
IF NOT EXISTS (SELECT * from sys.database_role_members WHERE USER_NAME(member_principal_id) = 'user1')
BEGIN
CREATE USER [user1] WITH PASSWORD = 'abc')
END
EXEC sp_addrolemember 'role1', 'user1'
GO
DBExample already has a user1, so when I try to run the script, SQL Server Management Studio complains about an 'Incorrect syntax near 'user1'. (in the create user line)
What am I missing to make this work?

Answer 1

I think you're confusing Logins with Users - in SQL Server 2008R2, at least, you can't have one without the other. I'd recommend having a quick look at Books Online for these concepts.
You're probably looking for something like:
IF NOT EXISTS (SELECT * from sys.server_principals WHERE name = 'user1')
BEGIN
CREATE LOGIN [user1] WITH PASSWORD = 'abc';
END
GO
USE DBExample
GO
IF NOT EXISTS
(
SELECT * from sys.database_principals dp
INNER JOIN sys.server_principals sp on dp.sid = sp.sid
WHERE dp.name = 'user1' or sp.name = 'user1'
)
BEGIN
CREATE USER [user1] FOR LOGIN [user1]
END
GO
IF NOT EXISTS (SELECT * from sys.database_role_members WHERE USER_NAME(member_principal_id) = 'user1')
BEGIN
EXEC sp_addrolemember 'role1', 'user1'
END
GO
This creates a Login if it doesn't exist, goes to the database then creates a User if it doesn't exist, then associates a User with a Role.

Answer 2

I used the answer found here: https://stackoverflow.com/a/6159882 to make use of variables to substitute the user name to get around the 'There is already login/user named xxx in the database' error SSMS was complaining about. The code looks like this at the end:
USE DBExample
GO
DECLARE #userName varchar(100)
SET #userName = 'user1'
IF NOT EXISTS (SELECT * from sys.server_principals WHERE name = #userName)
BEGIN
DECLARE #LoginSQL varchar(200);
SET #LoginSQL = 'CREATE LOGIN ' + #userName + ' WITH PASSWORD = abc';
EXEC (#LoginSQL);
END
IF NOT EXISTS (SELECT * from sys.database_principals WHERE name = #userName)
BEGIN
DECLARE #UserSQL varchar(200);
SET #UserSQL = 'CREATE USER ' + #userName + ' FOR LOGIN ' + #userName;
EXEC (#UserSQL);
END
IF NOT EXISTS (SELECT * from sys.database_role_members WHERE USER_NAME(member_principal_id) = #userName)
BEGIN
EXEC sp_addrolemember 'role1', #userName
END
GO