CREATE USER testusername WITHOUT LOGIN

CREATE LOGIN testl
    WITH PASSWORD = 'test'

CREATE USER test
    FOR LOGIN testl


GRANT SELECT, INSERT, UPDATE ON Doctor TO test

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED


BEGIN TRANSACTION
    EXECUTE AS USER = 'test'
        SELECT *
            FROM Doctor

        INSERT INTO Doctor VALUES ('Johnson', 'Jack',2)

        UPDATE Doctor SET Dep_ID=2 WHERE Doc_ID=71

        SELECT *
            FROM Doctor

Cannot grant, deny, or revoke permissions to sa, dbo, entity owner, information_schema, sys, or yourself.

Msg 15151, Level 16, State 1, Line 2
Cannot drop the user 'test', because it does not exist or you do not have permission.
Msg 15151, Level 16, State 1, Line 3
Cannot drop the login 'testl', because it does not exist or you do not have permission.