We like to enforce business rules at the database level wherever
we can, for the simple reason, particularly the business we are in, most database update happens
outside the end-user application layer.
That is not to say you shouldn't enforce at the application level too, but that the database is the last
line of defense, is usually more self-documenting than application code can be, and also protects you from your
programmers, even when that your programmers is you.
Domains are objects that you will find in many high-end
standards-compliant databases. They exist in SQL Server, Oracle, IBM Db2, Firebird, and PostgreSQL to name a few.
Domains have existed for a really long time in PostgreSQL. In PostGIS topology, Sandro Santilli (usually known as strk), takes advantage of them for fleshing out the topology support, and I got turned on to them by him.
With that said - let's dive into domains.
What are domains?
Domains are essentially a reusable packaging of check constraints. You use them as if they were a custom data type.
The nice thing about them is that they are usually transparent to applications that
don't understand them.
Example 1: Enforce pay ending/pay day happens only on certain days of the week
Here is an example -- suppose you had a payment system, and you had a rule that the pay thru end date has to
fall on a Friday. You could create a domain such as the following:
CREATE DOMAIN dom_payday
CONSTRAINT check_dow CHECK (trim(to_char(VALUE, 'day')) = 'friday');
COMMENT ON DOMAIN dom_payday IS 'Company payday rules';