CREATE TABLE table_name_12 (began_operation NUMBER, length__km_ VARCHAR2, stations VARCHAR2)

What is the average operation beginning with a length of ampang and over 27 stations?
SELECT AVG(began_operation) FROM table_name_12 WHERE length__km_ = "ampang" AND stations > 27
SELECT AVG("began_operation") FROM "table_name_12" WHERE "ampang" = "length__km_" AND "stations" > 27