Schema DDL: placeholder for supporting migration cases

[chuwy]

Migrated from snowplow-archive/schema-guru#140

[chuwy]

From original ticket:

Here I'm collecting possible Schema-migrations and ways to handle it

1. New optional property (column) added. Addition. Will be implemented in 0.6.0.

ALTER TABLE ADD COLUMN ... NULL

2. Property become nullable. Revision. See igluctl: support REVISION migration for properties becoming nullable #238

ALTER TABLE ADD COLUMN customer_id_new string null;
INSERT INTO customer_id_new SELECT from customer_id;
ALTER TABLE DROP COLUMN customer_id;
ALTER TABLE RENAME COLUMN customer_id_new TO customer_id
END TRANSACTION;

3. Property become union-type: string -> [string, number]. Revision. See Schema DDL: support REVISION migration into string/number union type #243

4. A default is added to a property (Addition)

5. A minimum and/or maximum value is removed from a numeric (Revision)

6. New required field added (Model)

7. Property widened (Addition)

[alexanderdean]

7. New required field added with default (Revision?)

[aldemirenes]

Migrated to snowplow/schema-ddl#32