Oracle - Adding NOT NULL constraint to a currently nullable column - sql

Question

I have a handful of numeric fields that are currently nullable. I want to add a NOT NULL constraint to these fields and have the default value now set to 0.
Data gets pushed in to this table on a schedule. There is nothing being pushed into these fields, and they're nullable, so these certain fields are all being set to null. This is causing some trouble when we're trying to do math with these fields.
If I first run updates on these fields to set all of the current nulls to zeros, should I have any issues adding a NOT NULL with default 0 constraint on these currently nullable fields? Is there anything I should look out for?
Thanks
edit:
SET SERVEROUTPUT ON
DECLARE
P_PROJECT_NUM VARCHAR2(200);
P_FISCAL_YEAR VARCHAR2(200);
P_RESP_CENTER VARCHAR2(200);
FIELD1 INTEGER;
FIELD2 INTEGER;
FIELD3 INTEGER;
BEGIN
P_PROJECT_NUM := '123456';
P_FISCAL_YEAR := '2019_2020';
P_RESP_CENTER := '123A';
FIELD1 := 0;
FIELD2 := 0;
FIELD3 := 0;
FMS_EXTRACT.LOAD_TEMPLATE(
P_PROJECT_NUM => P_PROJECT_NUM,
P_FISCAL_YEAR => P_FISCAL_YEAR,
P_RESP_CENTER => P_RESP_CENTER,
FIELD1 => FIELD1,
FIELD2 => FIELD2,
FIELD3 => FIELD3
);
COMMIT;

Answer 1

You can do it like this
Add the default clause to the columns
Update the null values to 0
Add the constraint NOT NULL
My example:
SQL> create table t ( c1 number , c2 number );
Table created.
SQL> insert into t values ( 1 , null ) ;
1 row created.
SQL> insert into t values ( null , 1 ) ;
1 row created.
SQL> select * from t ;
C1 C2
---------- ----------
1
1
SQL> alter table t modify c1 number default 0 ;
Table altered.
SQL> alter table t modify c2 number default 0 ;
Table altered.
SQL> update t set c1 = 0 where c1 is null ;
1 row updated.
SQL> update t set c2 = 0 where c2 is null ;
1 row updated.
SQL> commit ;
Commit complete.
SQL> alter table t modify c1 number not null ;
Table altered.
SQL> alter table t modify c2 number not null ;
Table altered.
SQL> insert into t ( c1 ) values ( 2 ) ;
1 row created.
SQL> select * from t ;
C1 C2
---------- ----------
1 0
0 1
2 0

Answer 2

This is too long for a comment. I don't think you want a NOT NULL constraint. That would result in your inserts failing if any of the NOT NULL columns are NULL.
The simplest solution is to not change the data at all. After all, NULL appears to be a valid value, if that is how the data is coming in. Just use COALESCE() in your logic:
select coalesce(x, 0) + coalesce(y, 0)
You can encompass this logic in a view or in generated columns:
alter table t add x_notnull generated always as (coalesce(x, 0));
If you want to prevent rows coming in with NULL values, then you can filter them out in your data important process. You don't provide details, so I can't make any suggestions.
You can add a default value. For instance:
alter table modify y default 0;
Then subsequent inserts where the value is missing will be replaced by the default value. However, explicit NULL values will not be affected.
Finally, you can add a NOT NULL constraint -- after removing all current NULL values.
Here is a db<>fiddle showing some examples of the above.