Among the crimes in all the districts in Chicago, what is the percentage of them happening in the Central district?
the Central district refers to district_name = 'Central'; percentage = divide(count(case_number where district_name = 'Central'), count(case_number)) * 100%
SELECT CAST(SUM(CASE WHEN T2.district_name = 'Central' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T1.case_number) FROM Crime AS T1 INNER JOIN District AS T2 ON T1.district_no = T2.district_no