Table schema:
CREATE TABLE TRANSACTIONS (transaction_type_code VARCHAR); CREATE TABLE Ref_Transaction_Types (transaction_type_description VARCHAR, transaction_type_code VARCHAR)

Question: Show the description of the transaction type that occurs most frequently.
SQL: SELECT T1.transaction_type_description FROM Ref_Transaction_Types AS T1 JOIN TRANSACTIONS AS T2 ON T1.transaction_type_code = T2.transaction_type_code GROUP BY T1.transaction_type_code ORDER BY COUNT(*) DESC LIMIT 1