SQL Cheat Sheet

Who has a role:

 select @@ServerName [Server Name], DB_NAME() [DB Name], u.name [DB Role], u2.name [Member Name]
    from sys.database_role_members m
    join sys.database_principals u on m.role_principal_id = u.principal_id
    join sys.database_principals u2 on m.member_principal_id = u2.principal_id
    where u.name = 'db_owner'
    order by [Member Name]

Who logged in as dbo:

#in user database run the command 
SELECT name, sid FROM sys.sysusers where name = 'dbo' . 
#in master database run the command 
SELECT name, sid FROM sys.sql_logins