oracle的substr函數的用法

oracle的substr函數的用法：
取得字符串中指定起始位置和長度的字符串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   結果是 te
select substr('emros',-3,1) value from dual      結果是 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'