oracle的substr函数的用法

     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‘