>> Basically I have 2 tables check-in and check-out where users check-in and check-out using an RFID card. I am wanting to display who's in the building or has been in the building on a given day. <<
Your design is fundamentally wrong. Your checking in and out as if they are separate attributes; they are attributes of a single event. Here is a simple skeleton for the table you should have. A copy of temporal queries in SQL by Rick Snodgrass is free as a PDF from the University of Arizona.
This design flaw is called attributes splitting. It means things that should be in one table or column have been modeled in multiple tables or multiple columns in the same table. A NULL end_timestamp means the event is still ongoing. You can use other constraints to prevent people from spending too much time in an event, simple arithmetic to determine how many hours the user spent at an event, and so forth when they're all in one place instead of being split like this
CREATE TABLE Events
(user_id CHAR(10) NOT NULL,
start_timestamp DATETIME2(0) NOT NULL,
CHECK(start_timestamp <= end_timestamp),
PRIMARY KEY (user_id, start_timestamp));
It would help if you would follow basic netiquette and post DDL. That's what we've done for over 30 years on SQL forums.
Please post DDL and follow ANSI/ISO standards when asking for help.