What is the invoice number and invoice date corresponding to the invoice with the greatest number of transactions?
SELECT T2.invoice_number ,  T2.invoice_date FROM Financial_transactions AS T1 JOIN Invoices AS T2 ON T1.invoice_number  =  T2.invoice_number GROUP BY T1.invoice_number ORDER BY count(*) DESC LIMIT 1
invoice with the greatest number of transactions refers to MAX(COUNT(transaction_id)); invoice number and invoice date corresponds to invoice_number and invoice_date respectively; JOIN operation is used to combine Financial_transactions and Invoices tables based on invoice_number;