主要的操作就是对文件名这个字符串进行操作： substring, reverse, position, split_part note:推荐方法二，简单
Method 1:SELECT REVERSE(SUBSTRING(REVERSE(name) FROM 1 FOR (POSITION(’.’ IN REVERSE(name))-1))) FROM directory_entry_file
REVERSE(name)：reverse 文件name, eg: a.china.cpp=> ppc.anihc.aPOSITION(’.’ IN REVERSE(name): 找到1st '.'位置 eg: 4SUBSTRING(REVERSE(name) FROM 1 FOR (POSITION(’.’ IN REVERSE(name))-1)): 获取 '.'前面的字符串 现在输出是 ppc, 再对他反转REVERSE(SUBSTRING(REVERSE(name) FROM 1 FOR (POSITION(’.’ IN REVERSE(name))-1)))Method 2:reverse(split_part(reverse(name), ‘.’, 1))
reverse(CONVERT_FROM(name, ‘latin1’)): 反转文件名, eg: a.china.cpp=> ppc.anihc.asplit_part(reverse(name), ‘.’, 1): 获取反转后’.'前面的第一部分 eg:ppc反转 “ppc” to “cpp” hart.js/