SQL SERVER: Map logins to dbs, and assign roles

AssIgnUserAndRoles.sql

--SELECT [sp].[name], [sp].[type_desc] FROM sys.[server_principals] AS [sp] WHERE type_desc LIKE 'windows%' ORDER BY [sp].[name]

-- SELECT * FROM sys.[server_principals] AS [sp] WHERE type_desc = 'WINDOWS_LOGIN' AND name LIKE '%.%'
DECLARE @username sysname = '****',
    @role_name sysname = 'db_owner'

IF NOT EXISTS (SELECT * FROM sys.[server_principals] AS [sp] WHERE [sp].[name] = @username) BEGIN
    RAISERROR('The user [%s] does not exist as a server login.', 16, 10, @username)
    RETURN;
END

DECLARE @sql VARCHAR(MAX) = ''
SELECT @sql += CONCAT('
    /****************************************************************************************/
    /****************************************************************************************/
    /****************************************************************************************/
    RAISERROR(''USE [', [d].[name], '];'', 0, 1) WITH NOWAIT;
    USE [', [d].[name], '];

    IF NOT EXISTS (SELECT 1 FROM [sys].[database_principals] AS [dp] WHERE [dp].[name] = ''', @username, ''') BEGIN
        RAISERROR(''CREATE USER [', @username, '] FOR LOGIN [', @username, '];'', 0, 1) WITH NOWAIT;
        CREATE USER [', @username, '] FOR LOGIN [', @username, '];
    END
    
    IF IS_ROLEMEMBER(''', @role_name, ''', ''', @username, ''') IS NULL BEGIN
        RAISERROR(''ALTER ROLE [', @role_name, '] ADD MEMBER [', @username, ']; '', 0, 1) WITH NOWAIT;
        ALTER ROLE [', @role_name, '] ADD MEMBER [', @username, ']; 
    END
') 
FROM sys.databases AS [d] 
WHERE d.[database_id] NOT IN (1, 2, 4) /* make changes to model so new dbs have the same rights */
ORDER BY [d].[name]

-- cast it to xml so the value can be clicked open into a window
SELECT CAST(@sql AS XML)
EXEC (@sql)