In our example – For one city, we could have many different customers located in it. When you want to determine the nature of the relation you need to establish between two tables just do this. And this works, because the customer can be only in one city and the city could have many different customers located in it. We simply established a relation from the city.id to customer.city_id. This the typical example of one-to-many relation and this is how we solved it in our model: For each customer, we also want to store the city where this customer is located, and we know that the customer will be in exactly one city. Imagine that we want to store a list of all our customers in the database. The reason for that is that it’s the most commonly used and the remaining two are “subtypes” of this one. The first of our 3 types of relations, we’ll start with is one-to-many. So, what’s different? We’ll explain each of these relations types separately and comment on what is their actual purpose. There are 3 different types of relations in the database:Īlthough they are different, they are represented in (almost) the same manner in the databases and that is the line between the two tables. Even between two lines (relations), we could have some minor differences. The question remains on how to relate two tables. We’ll use this relation when we need data from both these tables, mostly when we’re writing select queries. employee.id) is related to the value from another table (e.g. Without much effort, you can easily notice that each table is connected/related to another table with exactly one line ( foreign key). This also stands for types of relations between tables. While the model itself has only 6 tables (real-life models could have hundreds of tables), it contains the most common rules we’ll meet in many other models as well. In the previous article, Learn SQL: SQL Scripts, we’ve extended the data model we’ve used so far by adding a few new tables and filling them with the data.

Understanding these relations is somehow natural and not so complex but is still essential in the database theory (and practice).

That stands for both – a process of designing a database model as well as when you’re analyzing your data. One of the most important things in databases is to understand the types of relations in the databases.

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