MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE weather (events TEXT, zip_code INTEGER);
INSERT INTO weather VALUES
  ('event1', 94041), ('event2', 94041), ('event3', 94041), (null, 94041), 
  ('event1', 95113), (null, 95113), ('event3', 95113), (null, 95113);

Records: 8  Duplicates: 0  Warnings: 0


SELECT zip_code, 
       AVG(events IS NULL) * 100 AS percentage
FROM weather
GROUP BY zip_code;


zip_code
percentage


94041
25.0000

95113
50.0000