Updating field value if it is null

Question

I think this is a simple question but I haven't touched in Oracle SQL for a while.

I want to update a table if a certain field value is null.

I'm not sure if I should use the "not exists" clause, and this is what I have so far:

update table_classroom
set cod_classroom = 'unknown'
where exists (select * from table_classroom where cod_classroom is null)

Is this correct?

Answer 1

Even simpler:

update table_classroom
set cod_classroom = 'unknown'
where cod_classroom is null

Answer 2

Perhaps you also want to omit future entries where cod_classroom is null. If this is the case you don't need to update the column manually but you can include this in your alter statement:

ALTER TABLE table_classroom MODIFY (cod_classroom NOT NULL DEFAULT 'unknown')