Salesforce SOQL To Get Case Owner In One Hit

Using Salesforce SOQL I can get the Owner's Id using the following:

SELECT Case.OwnerId 
FRO开发者_StackOverflowM Case 
WHERE Case.CaseNumber = '00001234'

I can then get the User details for the User who owns the case in this query:

SELECT User.Id, User.Name, User.Custom_Field__c 
FROM User 
WHERE User.Id = '001A0000001abc1DEF'

But I can't get it to work in a single statement, I think this is because Owner != User, even though the owner is in fact a user in this case. I have tried:

SELECT Owner.Custom_Field__c
FROM Case 
WHERE Case.CaseNumber = '00001234'

But I get an error that the Custom_Field__c is not a valid field.

Owner on Case is a polymorphic relationship, it can be one of multiple different types. When that happens then using SOQL-R you can only select a subset of fields that are common to the types being pointed to (these are all exposed on a pseudo entity called "Name"), so you won't be able query the custom fields on User. You'd have to do a query on Case, then collect up the set of Owners and make a query to User and/or Group to the get more detailed information.

You can do this with a semi-join in SOQL. I tested it out, and it still works even though Owner is polymorphic:

SELECT Custom_Field__c 
FROM User 
WHERE Id IN (SELECT OwnerId 
             FROM Case
             WHERE Case.CaseNumber = '00001234')