In designing a relational database schema, many people will automatically create a column id integer primary key for every table, using their database’s automatic increment feature to assign a new value to each row they insert. My assertion is that this choice of primary key should be the last resort, not the first.
A database schema is a design artifact, describing the data we want to store and the relationships between records (rows) in those data. It is also meta-design, because the schema constrains us in designing the queries we use to work with the data. Using the same, minimal-effort primary key type for every table then avoids communicating information about the structure and meaning of the data in that table and imposes irrelevant features in the queries we design.
The fact that people use the name id for this autoincrementing integer field gives away the fact that the primary key is used to identify a row in a database. The primary key for a table should ideally be the minimal subset of relevant information that uniquely identifies an individual record. If you have a single column, say name, with not null and unique constraints, that’s an indicator (though not a cast-iron guarantee) that this column may be the table’s primary key. Sometimes, the primary key can be a tuple of multiple columns. A glyph can be uniquely identified by the tuple (character, font, swash) for example (it can, regardless of whether this is how your particular favourite text system represents it, or whether you think that this is a weird way to store ligatures). The glyphs “(e, Times New Roman Regular 16pt, normal)” and “(ct, Irvin Demibold 24pt, fancy)” are more readily recognisable than the glyphs “146276” and “793651”, even if both are ways to refer to the same data. A music album is identified by the artist and the album name (he says, side-eyeing Led Zeppelin): “A Night at the Opera” is ambiguous while “(Blind Guardian, A Night at the Opera)” is definitely not “(Queen, A Night at the Opera)”.
Use an integer identifier where there is no other way to uniquely identify rows in a table. Note: sometimes there is another, more meaningful way, even where that just means using somebody else’s unique identifier: different copies of the same book will have unique shelfmarks if they’re part of a library, for example. People in an organisation may have an employee number, or a single sign-on user name; though there may be privacy reasons not to use these.
A side-effect of using useful information to identify rows in a database is that it can simplify your queries, because where your foreign keys would otherwise be meaningless numbers, they now actually carry useful information. Here’s an example, from a real application, in which I’m sad to say I designed both the “before” and “after” schemata.
The app is a risk management tool. There are descriptions of risks (I’d like to believe that they all at least have a distinct description but I can’t be sure, so those will use integer id PKs), and for each risk there are people in certain roles who bring particular skills to bear on mitigating the risk. The same role can be applied to more than one risk, the same skill can be applied by more than one role, and one role may apply multiple skills, so there’s a three-way join to work out, for a given risk, what roles and skills are relevant.
The before schema:
create table risk (id integer primary key, description varchar not null, weight integer, severity integer, likelihood integer); -- many fields elided
create table role (id integer primary key, name varchar not null, unique(name)); -- ruh roh
create table skill (id integer primary key, name varchar not null, unique(name)); -- the same anti-pattern twice
create table risk_role_skill (id integer primary key, risk_id integer, role_id integer, skill_id integer, foreign key(risk_id) references risk(id), foreign key(role_id) references role(id), foreign key(skill_id) references skill(id));
In this application, we start by looking at a list of risks then inspect one to see what roles are relevant to mitigating it, and then what skills. So a valid question is: “given a risk, what roles are relevant to it?”
select distinct role.name inner join risk_role_skill on role.id = risk_role_skill.role_id where risk_role_skill.risk_id = ?;
But if we notice the names of each role and skill are unique, then we can surmise that they are sufficient to identify a given role or skill. In fact, the only information we have about roles or skills are the names.
create table risk (id integer primary key, description varchar not null, weight integer, severity integer, likelihood integer); -- many fields elided
create table role (name varchar primary key); -- uhhh...
create table skill (name varchar primary key); -- this still looks weird...
create table risk_role_skill (id integer primary key, risk_id integer, role_name varchar, skill_name varchar, foreign key(risk_id) references risk(id), foreign key(role_name) references role(name), foreign key(skill_name) references skill(name));
Here’s the new query:
select distinct role_name from risk_role_skill where risk_id = ?;
We’ve removed the join completely!
Two remaining points:
- There’s literally no information carried in the
role and skill tables now, other than their identifying names. Does that mean we need the tables at all? In this case no, but in general we need to think here. How are the names in the join table going to get populated otherwise? If there are a limited set of valid values to choose from, then keeping a table with the range of values and a foreign key constraint to that table may be a good way to express the intent that the column’s content be drawn from that range. As an example, a particular bookstore may have printed, ebook, and audiobook media, so could restrict the medium field in their stock table to one of those values.
- Why does the
risk_role_skill table have an identifier at all? It is a collection of associations between values, so a row’s content is that row’s identity.
Here’s the after schema:
create table risk (id integer primary key, description varchar, weight integer, severity integer, likelihood integer); -- many fields elided
create table risk_role_skill (risk_id integer, role varchar, skill varchar, foreign key(risk_id) references risk(id), primary key(risk_id, role, skill));
And the after query:
select distinct role from risk_role_skill where risk_id = ?;
Two fewer tables, no joins, altogether a much simpler database to understand and work with.
