CREATE TABLE accounts (account_id DECIMAL, statement_id DECIMAL, account_details TEXT)
CREATE TABLE statements (statement_id DECIMAL, statement_details TEXT)
CREATE TABLE documents_with_expenses (document_id DECIMAL, budget_type_code TEXT, document_details TEXT)
CREATE TABLE projects (project_id DECIMAL, project_details TEXT)
CREATE TABLE ref_budget_codes (budget_type_code TEXT, budget_type_description TEXT)
CREATE TABLE documents (document_id DECIMAL, document_type_code TEXT, project_id DECIMAL, document_date TIME, document_name TEXT, document_description TEXT, other_details TEXT)
CREATE TABLE ref_document_types (document_type_code TEXT, document_type_name TEXT, document_type_description TEXT)

Give the budget type code that is most common among documents with expenses.
SELECT budget_type_code FROM documents_with_expenses GROUP BY budget_type_code ORDER BY COUNT(*) DESC LIMIT 1
SELECT "budget_type_code" FROM "documents_with_expenses" GROUP BY "budget_type_code" ORDER BY COUNT(*) DESC NULLS LAST LIMIT 1