Show all the budget type codes for documents with expenses.
select budget_type_code from documents_with_expenses
For each code, show the number of documents.
select budget_type_code, count(*) from documents_with_expenses group by budget_type_code
Sort the codes in descending order of the count.
select budget_type_code from documents_with_expenses group by budget_type_code order by count(*) desc
Which has the most?
select budget_type_code from documents_with_expenses group by budget_type_code order by count(*) desc limit 1