Wherever you go, databases use SQL for queries: https://en.wikipedia.org/wiki/SQL#:~...ystem%20(RDSMS
If you know SQL one place you will be fine everywhere. This includes SELECT and action queries (e.g. INSERT, APPEND, DELETE). The different databases do have slight differences in the native functions (e.g. Now()=Access, getdate()=SQL Server, SYSDATE = Oracle), so that's usually where you will spend your time converting.
Writing functions/procedures use completely different languages, so any custom functions you have would have to be rewritten to whatever database you are moving.
To move queries between systems I would copy in the working SQL from the old system into a query window of the new system and see if it runs. If not, it will help you debug the query to get it working on the new system.