One-to-One, One-to-Many Table Relationships in SQL Server
Learn how to handle many-to-many relationships in database To create one for this example, we can create a new table called class_enrolment. As simple as SQL: Don Chamberlin shows how to analyze JSON data with. create table movie_category_junction (movie_id int, category_id int, CONSTRAINT The many to many relationship with categories is a good example of this. Many-to-many relationships between tables are accommodated in databases by means of junction tables. A junction table contains the primary key columns of the two tables you want to relate. To create a many-to-many relationship between tables in dbForge Studio for MySQL.
Now we have created our books and reviews tables, let's add some data to them. Since a column in reviews references data in books we must first ensure that the data exists in the books table for us to reference.
We set up the table in this way for our example because we wanted to focus on the one-to-many relationship type. If we had added such a Foreign Key to reviews we'd effectively be setting up a Many-to-Many relationship between books and users, which is what we'll look at next. Many-to-Many A many-to-many relationship exists between two entities if for one entity instance there may be multiple records in the other table, and vice versa.
A user can check out many books. A book can be checked out by many users over time. In order to implement this sort of relationship we need to introduce a third, cross-reference, table. We already have our books and users tables, so we just need to create the cross-reference table: Each row of the checkouts table uses these two Foreign Keys to create an association between rows of users and books.
Map Many-to-Many Relationships (Visual Database Tools) - SQL Server | Microsoft Docs
We can see on the first row of checkouts, the user with an id of 1 is associated with the book with an id of 1. On the second row, the same user is also associated with the book with an id of 2.
On the third row a different user, with and id of 2, is associated with the same book from the previous row. On the fourth row, the user with an id of 5 is associated with the book with an id of 3.
Don't worry if you don't completely understand this right away, we'll take a look shortly at what these associations look like in terms of the data in users and books. First, let's create our checkouts table and add some data to it. While these aren't necessary to create the relationship between the users and books table, they can provide additional context to that relationship.
Attributes like a checkout date or return date don't pertain specifically to users or specifically to books, but to the association between a user and a book. This kind of additional context can be useful within the business logic of the application using our database. Now that we have our checkouts created, we can add the data that will create the associations between the rows in users and books.
We can perhaps think of a Many-to-Many relationship as combining two One-to-Many relationships; in this case between checkouts and users, and between checkouts and books.
How to: Create many-to-many table relationships
Summary In this chapter we covered a number of different topics regarding table relationships: We briefly covered normalization, and how this is used to reduce redundancy and improve data integrity within a database. ERDs were introduced, and we discussed how these diagrams allow us to model the relationships between different entities. We also looked at keys, and how Primary and Foreign keys, and how these work together to create the relationships between different tables.
Finally we looked at some of the different types of relationships that can exist between tables and how to implement these with SQL statements. An example is a many-to-many between magazines and subscribers.
Really that link is a subscription with its own attributes, like expiration date, payment status, etc.
Creating multiple tables and table relationships
However, I think sometimes a linking table is just a linking table. The many to many relationship with categories is a good example of this. So in this case, a separate one field primary key is not necessary.
You could have a auto-assign key, which wouldn't hurt anything, and would make deleting specific records easier. It might be good as a general practice, so if the table later develops into a significant table with its own significant data as subscriptions it will already have an auto-assign primary key.
You can put a unique index on the two fields to avoid duplicates. This will even prevent duplicates if you have a separate auto-assign key. You could use both fields as your primary key which is also a unique index.
So, the one school of thought can stick with integer auto-assign primary keys, and avoids compound primary keys.