To do that you'd have to include a "selection filter" with each DB access from the client app:
... WHERE Company='Jones Marketing' AND ...;
That's a problem, because you are technically exposing one companies data to all companies in you deal with this at the client end: it isn't difficult to see the raw traffic and that you are specifically extracting client based data. Depending on what the data contains, you could easily fall foul of GDPR and the fines for "inappropriate" data can be considerable even if you don't directly reveal it yourself, as you are not taking due precautions to protect sensitive material.
You could do it at the server end: The client logs into to a server application which filters information specifically for the logged in user and cannot send any other data - and that's the only access route to the DB.
I'd rethink the whole design, if I was you ...
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