oracle的substr函数:取得字符串中指定起始位置和长度的字符串

oracle的substr函数的用法
 取得字符串中指定起始位置和长度的字符串   substr( string, start_position, [ length ] )
 如:

substr('This is a test', 6, 2)     would return 'is'
substr('This is a test', 6)     would return 'is a test'
substr('TechOnTheNet', -3, 3)     would return 'Net'
substr('TechOnTheNet', -6, 3)     would return 'The'
select substr('Thisisatest', -4, 2) value from dual    --结果是   te
select substr('emros',-3,1) value from dual      --结果是 r

substr('abcde',-6) = null
substr('abcde',-5) = 'abcde'
substr('abcde',-4) = 'bcde'
substr('abcde',-3) = 'cde'
substr('abcde',-2) = 'de'
substr('abcde',-1) = 'e'
substr('abcde',-0) = 'abcde'