Update rows using replace in SQL*Plus

For Example

SQL> desc rk
Name Null? Type
----------------------------------------- -------- ----------------------------
CONTENT_VALUE_SMALL VARCHAR2(512 CHAR)

SQL> select * from rk;

http://i.a.rk.net/v5cache/TBS/veryf/images/i0/3telecom_expo_400_304.jpg

Now you want to replace rk.net with xyz.net

SQL> update rk set CONTENT_VALUE_SMALL=replace(CONTENT_VALUE_SMALL,'http://i.a.rk','http://i.a.xyz');