Add ON DELETE CASCADE behavior to an sqlite3 table after it has been created

SQLite's ALTER TABLE command cannot do what you want.

However, it is possible to bypass the SQL interpreter and change the internal table definition directly. SQLite stores table definitions as a textual copy of the CREATE TABLE command in its sqlite_master table; check out the result of this query:

SELECT sql FROM sqlite_master WHERE type='table' AND name='skills';

Add your cascade specification to that string, then enable write access to sqlite_master with PRAGMA writable_schema=1; and write your new table definition into it:

UPDATE sqlite_master SET sql='...' WHERE type='table' AND name='skills';

Then reopen the database.

WARNING: This works only for changes that do not change the on-disk format of the table. If you do make any change that changes the record format (such as adding/removing fields, or modifying the rowid), your database will blow up horribly.