What is an SQL relation?


If you’re like me, understanding SQL is taking you a long time. And some efforts. It took me years to get a good mental model of how SQL queries are implemented, and then up from the lower levels, to build a mental model of how to reason in SQL.

Nowadays, in most case, I can think in terms of SQL and write queries that will give me the result set I need. In most cases, because some parts of SQL are still complex, and I don’t practice as much as I used to. In particular, newer PostgreSQL features are areas I still need to work with more before I can integrate them in my daily workflow.

Today, I want to introduce the central concept of the SQL language. What is an SQL relation?

The term comes from mathematics and their relational theory. The wikipedia page Relation (Mathematics) include an introduction about what is a relation… which uses a jargon that you might not be familiar with. At least I know I have a hard time understanding that page. So I’ll simplify it for you now.

In SQL, a relation is a bag of objects that all share the same characteristics: a list of attributes with a known given data type.

We name those objects tuples in SQL. An object with three attributes can be named a triple, an object with four attributes a quadruple, with six attributes a sextuple, etc. The generic that applies to any number of attributes is built on the same mechanism. Let’s say that we have the unknown number t of attributes. Then we’ll name the object a T-UPLE, or a tuple.

If you have ever worked with a collection of objects in Java, or Python, or Ruby, or PHP, or some other programming language, then you have already woked with a relation.

The most common relation in SQL is a table, and that’s why we have the TABLE command. The truth is that any SQL query defines a new relation. The result set of a SQL query is always a collection of tuples.

SQL provides different ways to compose relations together: set operators and join operators are used for that. Our next article will dive in SQL JOINs.

Meanwhile, check out my book The Art of PostgreSQL where you can learn how to put SQL to good use when coding your application!