Why does testing a business rule need a database?

One account expired yesterday; another expires tomorrow. Why should checking who gets a reminder require an email service?

Put the inputs on the table

Bernhardt moves the decision into a function. The outer code reads records, supplies them to the function and sends the resulting messages. Two records and a date are enough to test the rule. A separate integration check verifies that the database actually supplies those records.

the expired users class makes all the decisions and the sweeper class has all the dependencies.

Midnight is an input too

Hickey suggests checking whether identical inputs produce identical outputs. A function that secretly reads the clock can disagree with itself across midnight. Pass the date explicitly and you can replay the decision.

a true functional interface, same input, same output.

When a small test needs several mocks, list the values the decision actually needs. Then test separately how the running system obtains those values.