Update...set...format

I am executing an UPDATE table_name SET field_name = value

where field_name is a substring of a larger field: SET SUBSTR(RECORD_DATA,51,8) =

SQL does not like this. Must the field_name be an actual named (defined) field in the table?

RECORD_DATA must be column of table_name.

If this is from another table then this table must be joined in query so Oracle knows where this comes from.

Thank you. One more question— does the SUBSTR function work with SET ?

yes.

Example
update customers set name = substr(surname,1,10) where …
and this
update customers set name = substr(name,1,10) where …

But not:
update customers set substr(name,1,10) = substr(surname,1,10) where …

Cheers.