Sales → Products 1 sale can exist out of multiple productsĭid we mention all relationships? There are four entities and each entity has a relationship with every other entity, so each entity must have three relationships, and also appear on the left end of the relationship three times.Products → Sales 1 product (type) can be purchased in multiple sales.Sales → Stores 1 sale can only be made in 1 store at the time.Stores → Sales in 1 store multiple sales can me made.Products → Stores 1 product (type) can be sold in multiple Stores.Stores → Products in 1 store there are multiple products.Stores → Customers, 1 store can receive multiple customers.Customers → Stores 1 customer can purchase in multiple Stores.Products → Customers 1 product can be purchased by multiple customers.Customers → Products 1 customer can buy multiple products.Sales → Customers 1 sale is always made by 1 customer at the time.

Customers → Sales 1 customer can buy something several times.You'll get a list like this: (please note that 'product' represents a type of product, not an occurance of a product) For example: How many customers belong to 1 sale? How many sales belong to 1 customer? How many sales take place in 1 store? Click to view figure First, you need to state for each relationship, how much of one side belongs to exactly 1 of the other side. The cardinality shows how much of one side of the relationship belongs to how much of the other side of the relationship. The relationship is the connection between the entities, just like in the real world: what does one entity do with the other, how do they relate to each other? For example, customers buy products, products are sold to customers, a sale comprises products, a sale happens in a store. The next step is to determine the relationships between the entities and to determine the cardinality of each relationship. 'Salespersons' also participate, and because salespersons are people, we need a salespersons entity. These are all entities that need to be included in your database.īut what other things are happening when selling a product? A customer comes into the store, approaches the salesperson, asks a question and gets an answer. The 'Store' is a location 'Sale' is an event 'Products' are things and 'Customers' are people. If the information you want to include doesn't fit into these categories, than it is probably not an entity but a property of an entity, an attribute.įor example, imagine that you are creating a website for a store, what kind of information do you have to deal with? In a store you sell your products to customers. Everything you could want to put in a database fits into one of these categories. These entities exist in four kinds: people, things, events, and locations. The types of information that are saved in the database are called 'entities'.

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