A short overview of databases

If you’re going to get started using Metabase, it’s helpful to know a few key database terms.

Tables

Fundamentally, databases are made up of one or more tables. Tables contain one or more columns and one or more row. A row is made up of cells, and each cell has a value that corresponds to the column it falls under.

Here’s an example of a table:

Here, the columns are Name and Age. The first row contains two cells, one with John and one with 25, corresponding to the Name and Age columns, respectively.

Columns

All the cells in a column contain the same type of information. For example, in the sample table above, the Name column contains names in each cell, while the Age column lists ages.

Each field has a type that describes what kind of data is stored in the field.

Columns vs fields

A note about columns and fields, as these terms can be used interchangeably:

• A field is an element for storing data (e.g., the PRODUCT_ID field stores identification codes for products).

• A column is a list of values, and most often a list of values from a single field (e.g., the PRODUCT_ID column stores values from the PRODUCT_ID field). A column can also, however, be a list of values from multiple fields. For example, a column might contain values from an expression that computes the difference of values from two different fields: a TOTAL_WITH_DISCOUNT column, for example, could take values from the DISCOUNT field and subtract them from values in the SUBTOTAL field, and list the difference.

In Metabase (and elsewhere) you’ll often see these two terms used interchangeably, as in most cases a column refers to data from a single field.

Keys

A primary key (also known as an entity key) is a field in a table that uniquely identifies each row. For example, imagine a car reservation app where you can book a car in advance. The ID of the reservation could be the reservation number, and no two reservations would share the same reservation number, allowing each reservation to be uniquely identified by its reservation number.

Example

Reservations Table

In the above table, the Reservation ID field is the ID (primary key).

Relationships between tables

Tables can contain references to other tables, which establishes a relationship between them.

Let’s expand the database of our hypothetical car booking app. We could have two tables: one for reservations (let’s call it Reservations) and one for customers, (we’ll call this one Customers).

To connect reservation data to the corresponding customer data, we’ll use a foreign key. A foreign key is a special kind of field in a table that references a column with matching values in a different table. Almost always, the field that the foreign key points to is the ID or primary key in the other table.

In this case, we’ll connect each record in the Reservations table to the record of the corresponding customer who made that reservation. To do this, we’ll create a Customer column in the Reservations table whose values match those of the ID column in the Customers table.

Reservations

Customers

While the built-in relationship between primary and foreign keys make things easier for database users, most databases will allow you to make a join between two tables even if there is no key relationship. Check out our guides on the different types of SQL joins and how to use joins in Metabase.

Now if we wanted to analyze our hypothetical app’s database with Metabase, we could ask a question, like: What’s the average age of all customers who made reservations in December of 2015?

To do this, we’d open up the Reservation table, add a filter to only look at reservations between December 1 and December 31, 2015, and select Average of…. To select the average of Age specifically, we’d put our foreign key to use and select Age from the Customers table that our Reservations table references.

Next: Data Types

Now that we have a basic understanding of databases, let’s learn more about data types.