试试这个：

select regexp_replace(first_name,'e','s',1,1) 
from your_table

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regexp_replace函数的解释如下：http://www.java2s.com/Book/Oracle/String_Functions/REGEXP_REPLACE_function.htm

另一个变体，不像regexp_replace那么优雅，但也可以工作：

with example_data as (
  select 'leena' field0 from dual union all
  select 'stev'  field0 from dual union all
  select 'neene' field0 from dual union all
  select 'ajay'  field0 from dual union all
  select 'vine'  field0 from dual 
)
select
  field0,
  decode( instr(field0,'e'),
          0, field0, 
          substr(field0,1,instr(field0,'e')-1) || 's' || substr(field0,instr(field0,'e')+1)) 
from 
  example_data

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SQLFiddle
只是为了说明。

Mostly we can use 'regexp_replace' to replace particular character from a particular occurrence.

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最简单的解决方案之一：

SELECT REPLACE(
                 SUBSTR(COLUMN_NAME, 1,
                        instr(COLUMN_NAME,'e',1)),
                 'e', 's')
         || SUBSTR(COLUMN_NAME,
                   INSTR(COLUMN_NAME,'e',1)+1,
                   LENGTH(COLUMN_NAME))
  FROM TABLE_NAME

型