Fundamentals of Data Analysis Relation types: one to one, one to many, many to many

36. Relation types: one to one, one to many, many to many

One to many

In the previous lesson, we saw a classic example of a one-to-many relationship: “a user has many purchases”. Every row in the purchases table has a column user_id which references a record in the users table with this id.

In SQL terminology user_id is an example of a foreign key – a column that points to another table.

id user_id amount created_at
11 2604 14,99 2019-01-02 21:00:29.390141
12 2866 4,99 2019-01-01 20:05:16.503062

This type of association is called one-to-many because each purchase belongs to a single user and...

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