[Last time](https://forum.azimuthproject.org/discussion/2210/lecture-37-chapter-3-presentations-of-categories/p1) we saw a graph that could be used for designing databases:



This graph is not a database; it's just a kind of template for a database. Fong and Spivak call it a **database schema**. To create an actual database using this schema, you'd have to specify:

* a set of employees
* a set of departments
* a set of strings (that is, strings of symbols)
* a function mapping each employee to the department they work in
* a function mapping each employee to their manager
* a function mapping each department to its secretary
* a function mapping each employee to their first name
* a function mapping each department to its name.

You could do this using a table or spreadsheet, or some other way. See the start of Chapter 3 for more on this!

What does this have to do with category theory? _A lot!_

We've seen that this "database schema" is really a graph \\(G\\). And we saw that from a graph we can get a category \\(\mathbf{Free}(G)\\) called the "free category on \\(G\\)". This has nodes of the graph as objects and paths as morphisms.

So, our database schema gives a category \\(\mathbf{Free}(G)\\). The objects in this category are things like \\(\textrm{Employee}\\) and \\(\textrm{Department}\\). The morphisms are things like \\(\textrm{WorksIn} : \textrm{Employee} \to \textrm{Department}\\).

To get an actual database, we need to provide a _set_ for each _object_ of \\(\mathbf{Free}(G)\\), and a _function_ for each _morphism_ of \\(\mathbf{Free}(G)\\).

When we do this, we're providing some sort of map from the category \\(\mathbf{Free}(G)\\) to the category \\(\textbf{Set}\\). A map between categories is called a "functor": it sends objects to objects, and morphisms to morphisms... and it should also obey a few obvious rules, which I'll explain in a second.

But first, enjoy the beauty of this idea! A database schema is really a category, while an actual database is a functor from that category to \\(\textbf{Set}\\). The database schema has "abstract" objects and morphisms like \\(\textrm{Employee}\\) and \\(\textrm{WorksIn} : \textrm{Employee} \to \textrm{Department}\\), while the functor makes these "concrete", turning them into actual sets and functions!

So what's a functor, exactly?

**Definition.** Given categories \\(\mathcal{C}\\) and \\(\mathcal{D}\\), a **functor** from \\(\mathcal{C}\\) to \\(\mathcal{D}\\), written \\(F: \mathcal{C} \to \mathcal{D} \\), consists of:

1. A function sending objects of \\(\mathcal{C}\\) to objects of \\(\mathcal{D}\\). This is usually called \\(F\\) as well, so we have \\(F: \mathrm{Ob}(\mathcal{C}) \to \mathrm{Ob}(\mathcal{D}) \\).

2. For each pair of objects \\(x,y \in \mathrm{Ob}(\mathcal{C})\\), a function sending each morphism \\(f: x \to y\\) to a morphism \\(F(f) : F(x) \to F(y) \\). As you can see, we're calling this function \\(F\\) too! So, it's a function

\[ F: \mathcal{C}(x,y) \to \mathcal{D}(F(x),F(y)) .\]

Remember, \\(\mathcal{C}(x,y)\\) is the set of morphisms from \\(x\\) to \\(y\\) in \\(\mathcal{C}\\).

We demand that:

a) \\(F\\) preserve composition: \\(F(g \circ f) = F(g) \circ F(f) \\) whenever \\(f\\) and \\(g\\) are morphisms that can be composed in \\(\mathcal{C}\\).

b) \\(F\\) preserves identities: \\(F(1_x) = 1_{F(x)}\\) whenever \\(x\\) is an object of \\(\mathcal{C}\\).

That's it!

It may freak you out that we're using \\(F\\) to mean the functor, the function that describes what this functor does to objects, _and_ all the functions that describe what \\(F\\) does to morphisms. However, this turns out not to be a problem. It would be much worse to have separate notations for all these things. You should think of them all as a single package: \\(F\\).

You may wonder about conditions a) and b). They seem to have shown up by magic. How do they arise in our study of databases?

Here's an example. Take \\(\mathcal{C} = \mathbf{Free}(G)\\) and look at the morphisms \\(\textrm{WorksIn} : \textrm{Employee} \to \textrm{Department}\\) and
\\(\textrm{DepartmentName} : \textrm{Department} \to \textrm{String}\\). Then choose a functor \\(F : \mathcal{C} \to \mathbf{Set}\\). We must have

\[ F(\textrm{DepartmentName} \circ \textrm{WorksIn}) = F(\textrm{DepartmentName}) \circ F(\textrm{WorksIn}) .\]

**Puzzle 109.** What does this mean, in practical terms?

There's a lot more to say, but not now!

**[To read other lectures go here.](http://www.azimuthproject.org/azimuth/show/Applied+Category+Theory#Chapter_3)**