I wouldn't write an 'app' that accepts SQL, but I've given out database access to clients who had software developers on-staff. It's a quick way to give access

I wouldn't write an 'app' that accepts SQL, but I've given out database access to clients who had software developers on-staff. It's a quick way to give access to data, and you can use permissions to restrict them. You'll want to give them their own private database. I wouldn't give out general access to a database to the wider internet. There are plenty of ways to cause trouble even with read-only access. Transactions and expensive queries are the first ones that come to mind.