Table schema:
CREATE TABLE student (lname VARCHAR, sex VARCHAR, city_code VARCHAR, age VARCHAR)

Question: Find the last name of students who is either female (sex is F) and living in the city of code BAL or male (sex is M) and in age of below 20.
SQL: SELECT lname FROM student WHERE sex = 'F' AND city_code = 'BAL' UNION SELECT lname FROM student WHERE sex = 'M' AND age < 20