前缀索引

select count(distinct left(business_code,14))/count(*)
, count(distinct left(business_code,15))/count(*)
, count(distinct left(business_code,16))/count(*)
, count(distinct left(business_code,17))/count(*) 
from member_score_change_detail_00;
+-------------------------------------------------+-------------------------------------------------+-------------------------------------------------+-------------------------------------------------+
| count(distinct left(business_code,14))/count(*) | count(distinct left(business_code,15))/count(*) | count(distinct left(business_code,16))/count(*) | count(distinct left(business_code,17))/count(*) |
+-------------------------------------------------+-------------------------------------------------+-------------------------------------------------+-------------------------------------------------+
|                                          0.7846 |                                          0.7857 |                                          0.7857 |                                          0.7858 |
+-------------------------------------------------+-------------------------------------------------+-------------------------------------------------+-------------------------------------------------+
1 row in set (2 min 52.01 sec)