select left(ds.start_time,7) as monthss,ds.start_time from debug_stock ds 用left，正确
select substring(ds.start_time,0,6) as monthss,ds.start_time from debug_stock ds，不报错，但是monthss为空
mysq截取字符串
原文：http://www.cnblogs.com/longsanshi/p/4904033.html
