Let's say you have an existing nullable column. You want to update the schema
to enforce a not null constraint on that column. You can do that with an
alter table DDL
statement. You can do this with the modify or change option.
With modify you redeclare the column definition with the options that you
want. You'll need to know and specify the existing data type of that column.
alter table books modify publication_year int not null;It is possible, but clumsy to do this with the change option because you
declare the column name twice. That's because change is typically used to
rename a column.
alter table books change publication_year publication_year int not null;If you're updating a column for a table that already contains data, make a plan
to backfill any existing records that have null for that column. Every record
will need a value in that column before the modify can be applied.