How do I update the substring of a string value?

Question

UPDATE table1
SET  SUBSTR  (TO_CHAR(field_value),3,2) = ' 50'  and SUBSTR (TO_CHAR(field_value),5,2)  =  '50'
where  ( SUBSTR(TO_CHAR(field_value), 3, 2) =  '100' ) or ( SUBSTR(TO_CHAR(field_value), 5, 2) =  '100' )tried the above in TOAD and I got "ORA-00927: missing equal sign." Please help!
data type for field_value is number. oracle version is 10g
thank you

Answer 1

Here's an example of how you can do it:
SQL> CREATE TABLE x
  2  AS
  3  SELECT 'SOME50HERE' AS field_x
  4  FROM dual;
Table created.
SQL> SELECT * FROM x;
FIELD_X
SOME50HERE
1 row selected.
SQL> UPDATE x
  2  SET    field_x =  SUBSTR(field_x,1,4)
  3                 || '75'
  4                 || SUBSTR(field_x,7)
  5  ;
1 row updated.
SQL> SELECT * FROM x;
FIELD_X
SOME75HEREBasically you need to break your string into three parts:
1. The part before the text you want to change.
2. The text you want to change.
3. The part after the text you want to change.
Then you concatenate them all together.
If #1 is not a known fixed number of characters, you can still use SUBSTR, by looking for the string you want to change like the following:
UPDATE x
SET    field_x =  SUBSTR(field_x,1,INSTR(field_x,'50') - 1)
               || '75'
               || SUBSTR(field_x,7)
;