Welcome to the community @David_King!
In SQL a field will always exist, but the value may be nullable. The closest equivalent to an $exists
query would be:
SELECT * FROM goods where product IS NOT NULL
For a general guide to roughly equivalent SQL queries, the MongoDB manual has some helpful reference pages:
Typically you will not use identical data models in MongoDB vs RDBMS. I included more on this in a discussion yesterday, so please read my related post for more details:
Regards,
Stennie