This is a common question, especially when customers upgrade from EE and they have saved queries that should be converted to advanced queries. It seems there is no information available for IFS Cloud advanced queries though this has been a topic from 21R1.  

Using basic search first, takes one only a bit forward. Would be interesting to know how nested queries work with advanced search, or what query syntax one should use with advanced query editor. Is it odata?

How to find customer orders based on customer country would be a good example of using advanced query, as the country attribute is not available in the Customer Order view/projection. 

As I know this is OData filtering.

Start with this:

https://learn.microsoft.com/en-us/azure/search/search-query-odata-filter

I'm looking for some user-friendly documentation, or even consultants who are less technical. But thanks for the link.

Upgrading from IEE to Cloud does not seem viable due to so many missing options (All saved searches will be wiped out, conditional formatting will be wiped out). Even if we ask end-users to set all their settings up from scratch (which is not how an upgrade should work!), it is still not possible to easily replicate saved searches to achieve the same results they can achieve with a few clicks in IEE.

We understand that underlying technology changes mean that the same functions are technically more challenging to achieve / uplift, but end users should not pay the price for technology changes. It should make their lives better to upgrade. Not worse. Right now, its the latter. 

We feel like Cloud teams designing/developing/releasing these features should put more thought and care about how existing customers already heavily invested in these features would be affected. :(

We’re moving from Apps8 to Cloud and having the same issues where some users have advance searches, some even making API.Get calls to other tables, and finding out this is not possible in Aurena

It’s not possible directly no, which is a shame. Basically you can’t just willy nilly run a select statement based on how IFS Cloud is architectured (this is on purpose). Any oData filtering you apply would need to be based on attributes that exist against the projection/entity you’re selecting from.

It is possible to create custom attributes that replicate your get commands and then use that for filtering, but it is not really a recommended approach as that would be a killer for performance (querying read only fetch attributes being generally quite slow unless the table you’re querying from has very few rows the get function needs to be ran on)