NULL entries are included when using the SQL aggregation function.

INSERT INTO customers (customer_id, customer_name, age, ranking, job, savings)
VALUES ('apple', 'Hong Gil-dong', 20, 'gold', 'student', 1000),
       ('banana', 'Kim Cheol-soo', 25, 'vip', 'Nurse', 2500),
       ('carrot', 'Park Young-hee', 28, 'gold', 'teacher', 4500),
       ('orange', 'Choi Yongwook', 22, 'silver', 'student', 0),
       ('melon', 'Sung Won-yong', 35, 'gold', 'office worker', 5000),
       ('peach', 'Oh Hyeongjun', NULL, 'silver', 'Doctor', 300),
       ('pear', 'glowers', 31, 'silver', 'office worker' 500);

SELECT COALESCE(SUM(age), 0) FROM customers;
> 185