Which are the most often used SQL queries? the question sounds pretty simple but I couldn't find anything on topic except for this article here. So is there any (even remotely) scientific (or at least accredited) opinion on the matter? The actual DBMS doesn't matter at all - I just want to know if joins outweigh selects in the long run, or inserts outweigh creates and (if possible) by what margin. Something like that.
It depends...

This is extremely specific to the environment.  There are two types:

OLAP (Online Analytical Processing) - this type of system is designed more for reporting and analysis, in which case often times SELECTs dominate the majority of the queries.

OLTP (Online Transaction Processing) - this type of system is designed more for data manipulation, and typically INSERT`UPDATE\DELETEstatements are significant enough to optimize for this type of transaction.SELECT` is also a large part of this environment, and considerations need to be made for this as well.

Often times these roles are encompassed by a single instance/database.  Because of this both must be considered.  There are dominant systems (like Data Warehouses) that may be dedicated soley to OLAP.


  I just want to know if joins outweigh selects in the long run


Joins simply create a relationship between tables.  They are used in conjuction with the respective DML statement (SELECT, INSERT, UPDATE, DELETE).


  or inserts outweigh creates


It sounds like you're asking the ratio of DDL vs. DML.  In a typical implementation DML will be the majority of a production environment's queries, though DDL is almost never going to be stale.