Question: Among the postal points in California, calculate the percentage of them in post office types.
SQL: SELECT CAST(COUNT(CASE WHEN T2.type = 'Post Office' THEN T2.zip_code ELSE NULL END) AS REAL) * 100 / COUNT(T2.zip_code) FROM state AS T1 INNER JOIN zip_data AS T2 ON T1.abbreviation = T2.state WHERE T1.name = 'California'