Sample Query:

SELECT Id, Name__c, AirDate__c 
FROM Advertisements__x 
WHERE Name__c LIKE '%Early%'

here Advertisements__x is an External object.

Check the below link for additional information

https://developer.salesforce.com/docs/atlas.en-us.soql_sosl.meta/soql_sosl/sforce_api_calls_soql_limits.htm

Note:

1. In SOQL queries of external objects, IN clauses with more than approximately 15 IDs return the error “This operation is too complicated for Secure Agent.” The exact IN clause limit varies based on SharePoint ID length.

2. A subquery that involves external objects can fetch up to 1,000 rows of data.