If you are a beginner for SQL DBA profile, an interviewer may as this question like What is the difference between SINGLE_USER, RESTRICTED_USER and MULTI_USER in SQL Server.

If you confuse and want to give your answer in one line, you can say like These are the ALTER DATABASE SET commands which we can use to get exclusive access to SQL Server Database.

SINGLE_USER: Only one user can connect to the database at a time

ALTER DATABASE Database_Name 
SET SINGLE_USER WITH ROLLBACK IMMEDIATE
GO

RESTRICTED_USER: Only DB_OWNER, DB_CREATOR or SYSADMIN roles can connect to the database

ALTER DATABASE Database_Name 
SET RESTRICTED_USER WITH ROLLBACK IMMEDIATE
GO

MULTI_USER: Any number of users can connect to the database

ALTER DATABASE Database_Name 
SET MULTI_USER 
GO