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# Back: SQL Analytics M7 - Many-to-Many Relationships and Entity Relationship Diagrams

A many-to-many relationship in databases occurs when multiple records in one table are associated with multiple records in another table. This kind of relationship often requires a third table, known as a junction, associative or joining table (depending on who you ask). For this class, I will use the term joining table. The joining table is used to break a many-to-many relationship down into two one-to-many relationships.

Let’s explore this concept with a new set of tables for books and authors that represent a many-to-many relationship.

In the BooksAuthors table, both BookID and AuthorID are used together to create unique combinations, allowing for the representation of multiple authors per book and vice versa.

1. First One-to-Many: Each record in the Authors table can be linked to multiple records in the BooksAuthors table. For example, AuthorID 2 is linked to BookID 102 and 103. This is a one-to-many relationship from Authors to BooksAuthors.

2. Second One-to-Many: Similarly, each record in the Books table can be linked to multiple records in the BooksAuthors table. For instance, BookID 103 is linked to AuthorID 1 and 2. This is another one-to-many relationship, but from Books to BooksAuthors.

3. The Many-to-Many: These two one-to-many relationships combine to form a many-to-many relationship. An author can write multiple books, and a book can be written by multiple authors. The BooksAuthors table serves as a bridge, linking each book to one or more authors and each author to one or more books.

# Back: SQL Analytics M7 - Many-to-Many Relationships and Entity Relationship Diagrams