Constraints are a way to ensure that certain properties hold in a relational database. You've already seen some examples of constraints: primary key constraints ensure non-nullity and uniqueness of an attribute, for example. However, a richer set of constraints can be defined and enforced over your database, giving you ease of mind. For example, we might like that other fields be non-null, unique, fit some domain. In this chapter, we'll explore a set of basic constraints that help us verify our data, as well as how one could write SQL to enforce these constraints.
We are primarily motivated to write constraints that verify the integrity of our data. In particular, there are four main kinds of integrity constraints: key constraints, attribute constraints, referential integrity constraints, and global constraints.
Key constraints, otherwise known as uniqueness constraints, ensure uniqueness over a set of attributes. One might write this constraint like:
CREATE TABLE ABC( a INT, b INT, c INT, PRIMARY KEY (a), UNIQUE (b, c));
PRIMARY KEY constraint ensures non-nullity and uniqueness, while the
UNIQUE constraint ensures uniqueness. In the above example, two rows
(1, 2, 3), (1, 4, 5) couldn't coexist, neither could two rows
(0, 2, 3), (1, 2, 3).
Attribute constraints ensure that values of a particular attribute fulfills some predicate. One could use this to enforce that an integer be positive or odd, or that a string start with a particular prefix. One might write this constraint like:
CREATE TABLE ABC( a INT, b INT, c INT, CHECK (c > 0));
Referential integrity constraints ensures that a particular attribute in one table references a primary key of another table. This ensures that no tuple is left "dangling" if their referential pair is deleted; indeed, this could lead to extra data not being cleaned out of the system. We've covered the
FOREIGN KEY <a> REFERENCES <b> already.
Global constraints are constraints that should apply to the whole database. In particular, they are generally phrased as a query that, when run, should always produce true. For example:
CHECK (NOT EXISTS( SELECT * FROM customers AS c, jobs as j WHERE c.jid == j.id AND c.cost < 0 ));
These are comparatively costly to run as they require an entire query to run every time a change is made to the database. However, they are incredibly powerful in ensuring integrity.