Many-to-many relationships

- The third and final type of relationship … that you can create … is a many-to-many relationship. … If you've been following along … then it's probably no surprise … that a many-to-many relationship … is created between two columns … where neither of them have a unique constraint applied. … In theory this is what a many-to-many relationship … looks like in a design diagram. … Here we have a database … that keeps track of class schedules … for a university. … Each student can be enrolled … in zero to multiple classes … and each class can have … between zero and many students. … I've noted the optionality and cardinality here … so we can clearly see the need … for a many-to-many relationship … based on the cardinality of N at both ends. … However, there isn't a good way … to relate the tables together … based (mumbles) the columns that we have. … To create the relationship according to the cardinality … we would need a non-unique column in each table. … The solution to this problem … is to simply not create a many-to-many relationship. …