CREATE TABLE Ref_Transaction_Types (transaction_type_description VARCHAR, transaction_type_code VARCHAR)
CREATE TABLE TRANSACTIONS (transaction_type_code VARCHAR)

Show the description of the transaction type that occurs most frequently.
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
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 NULLS LAST LIMIT 1