5.46Scenario 5 — Cases

Retrieve open high-priority Cases for customer C001.

Conceptually:

SELECT
    Id,
    CaseNumber,
    Subject,
    Status,
    Priority,
    Contact.Name
FROM Case
WHERE Account.External_Customer_ID__c = :customerId
AND Priority = 'High'
AND IsClosed = FALSE

Now you sound perfectly comfortable traversing Salesforce relationships.