To grant db_reader/db_writer permission to all or particular user to DB by bulk /single

----------------------To grant db_reader permission to all user for particular DB by bulk

DECLARE @name VARCHAR(50) -- database name 

DECLARE @user sysname
DECLARE @login sysname
DECLARE @SQL nvarchar(max) = ''

DECLARE db_cursor CURSOR READ_ONLY FOR 

 select name from sys.syslogins where name not in ('sa','dbadmin') and name not like '%#%' and isntname=0

OPEN db_cursor 
FETCH NEXT FROM db_cursor INTO @name 

WHILE @@FETCH_STATUS = 0 
BEGIN 

SET @login=@name
SET @user=@name

SELECT @SQL = @SQL + '
    USE ' + QUOTENAME(NAME) + ';

    IF NOT EXISTS(SELECT 1 FROM sys.database_principals WHERE name = ''' + @user + ''')
    BEGIN
        CREATE USER ' + QUOTENAME(@user)
           + ' FOR LOGIN ' + QUOTENAME(@login)
           + ' WITH DEFAULT_SCHEMA=[dbo];
    END
    ELSE
    BEGIN
        EXEC sp_change_users_login ''Update_One'', ''' + @user + ''', ''' + @login + ''';
    END

    EXEC sys.sp_addrolemember ''db_datareader'', ''' + @user + ''';

   
'
FROM sys.databases
WHERE database_id > 4 AND state = 0 and database_id=15

EXEC sp_executesql @sql;
print @sql

   FETCH NEXT FROM db_cursor INTO @name 
END 

CLOSE db_cursor 
DEALLOCATE db_cursor

----------------------To grant db_reader permission  particular user for particular /all DB


DECLARE @user sysname = 'testpub123';
DECLARE @login sysname = 'testpub123';
DECLARE @SQL nvarchar(max) = '';

SELECT @SQL = @SQL + '
    USE ' + QUOTENAME(NAME) + ';

    IF NOT EXISTS(SELECT 1 FROM sys.database_principals WHERE name = ''' + @user + ''')
    BEGIN
        CREATE USER ' + QUOTENAME(@user)
           + ' FOR LOGIN ' + QUOTENAME(@login)
           + ' WITH DEFAULT_SCHEMA=[dbo];
    END
    ELSE
    BEGIN
        EXEC sp_change_users_login ''Update_One'', ''' + @user + ''', ''' + @login + ''';
    END

    EXEC sys.sp_addrolemember ''db_datareader'', ''' + @user + ''';

   
'
FROM sys.databases
WHERE database_id > 4 AND state = 0 and database_id=24;

EXEC sp_executesql @sql;
print @sql

--select * from sys.sysusers

--sp_helpdb Demo_DB1

--select * from sys.databases