oracle数据库的substr方法 各种参数的意思。

substr( string, start_pos, [ 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'

substr('a123456',instr('a123456'，'a')+1,len('a123456')-instr('a123456','a'))

from

dual

instr

返回1，

所以给它+1

，

从第2位开始截取到总长度-‘a’这个字符串的位置，

就是

6

，

所以最终会是

select

substr('a123456',2,6)

from

dual

这么写能明白吗？