Database: advertising_agencies
Question: Return the ids and details of clients who have attended a meeting or had an invoice.
SQL:
SELECT T1.client_id ,  T1.client_details FROM Clients AS T1 JOIN meetings AS T2 ON T1.client_id  =  T2.client_id UNION SELECT T1.client_id ,  T1.client_details FROM Clients AS T1 JOIN invoices AS T2 ON T1.client_id  =  T2.client_id