Task: Text to Code
Input (English): Step 1: For each row in Charges table, find corresponding rows in Treatment_Types table and in Treatments table.
Step 2: find the average charge_amount of each value of Treatments's treatment_type_code in the results of step 1.
Step 3: find treatment_type_description of the results of step 1 with smallest value in the results of step 2
Output (English): SELECT T2.treatment_type_description FROM Charges AS T1 JOIN Treatment_Types AS T2 JOIN Treatments AS T3 GROUP BY T3.treatment_type_code ORDER BY Avg ( T1.charge_amount ) Asc LIMIT 1