Variable Constraint Messages: Example 1
In the Travel Agency, the business rule: "Guides must have a bank account" is implemented by a restrictive, single-record constraint:
SELECT first_name, last_name
WHERE guide = 'Y'
AND bank_account IS NULL
Because the first_name and last_name columns are specified in the SELECT clause, you now can specify the message:
<person.first_name.value> <person.last_name.value> is a guide and must have a bank account.
The error message displayed will be:
"John Smith is a guide and must have a bank account".
You can also refer to alias names and labels.