Verify zero data loss on migration

Move customer addresses into a new customer_addresses table and prove, with counts and two-way set differences, that no row or value was lost.

1. 1Create customer_addresses (customer_id INTEGER PRIMARY KEY REFERENCES customers(id), city TEXT, country TEXT NOT NULL).
2. 2Copy id, city, country from customers; three customers have a NULL city and must survive the copy.
3. 3Compute source_rows, target_rows, missing_rows (customers EXCEPT customer_addresses) and unexpected_rows (the reverse).
4. 4Return them as one row: SELECT (SELECT COUNT(*) FROM customers) AS source_rows, … AS target_rows, … AS missing_rows, … AS unexpected_rows.

• customer_addresses holds every customer's city and country
• The last result set is (30, 30, 0, 0)
• The verification uses EXCEPT

• Every run starts from a freshly seeded database, so your script must include its own setup statements.
• Only the last result set of your script is compared, so finish with the verification query.