Are there best practices or standardized syntax for writing transformation logic?
SQLish

Honestly, your best bet is probably ANSI-compliant SQL, because it can eliminate the ambiguities which you've mentioned.

If there's any trouble with it needing to be written in "natural language", just add in the apprpriate prepositions and such...

Select the maximum paid amount, the customer account number, and the customer phone number 
from the 'Invoices' table joined to the 'Customers' table, 
joined on the condition that 
the 'Invoices'.customer_id column equals the 'Customers'.customer_id column, 
grouping by the customer account number and customer phone number.