[muloka]

RethinkDB is still a strong possibility as an alternative, it's similar enough to MongoDb.

While there is no official core SQL support yet there are 3rd party add ons for PostgreSQL and MySQL available.

[zzzmarcus]

There are no Postgres or Mysql add-ons for Meteor that are actively maintained or even close to production ready. Hopefully you can prove me wrong on this, but I've looked and have only found preview releases and alpha quality packages.

[muloka]

This looks promising https://github.com/jordangarside/Meteor-GraphQL-PostgreSQL-D...

[aioprisan]

Today, there are 0 add-ons that support MySQL, that are actively maintained or worked on, that you can download through atmosphere, the package manager.