Oracle函数 substr

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