-- Which customer's portfolio a conversation is about, so reopening it (on any device) picks up where it left off.
-- Plain columns, not a foreign key: customers live in the portfolio database, not this one. The name is kept so the
-- conversation can show who it is about without asking the portfolio database.
ALTER TABLE conversations
  ADD COLUMN customer_id   INT UNSIGNED NULL AFTER title,
  ADD COLUMN customer_name VARCHAR(255) NULL AFTER customer_id;