At times I've done crude implementations of similar functionality, by basically just taking the current string on change and concatenating with " LIMIT 20" before passing it to the database API and then rerendering a table if the result is an associative array rather than an error message.

I think this would be better if it was combined with information about valid words in the cursor position, which would likely be a bit more involved but achievable through querying the schema and settling on a subset of SQL. It would help people that aren't already fluent in SQL to extract the data they want. Perhaps allow them to click the suggestions to add them to the query.

I've done partial implementations of this too, that query the schema for table or column names. It's very cheap even on large, complex schemas, so it's fine to just throw every change at the database and check what drops out. In practice I didn't get much out of either beyond the fun of hacking up an ephemeral tool, or I would probably have built some small product around it.