Weak entity

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In a relational database, a weak entity is an entity that cannot be uniquely identified by its own attributes alone; therefore, it must use a foreign key in conjunction with its attributes to create a primary key. The foreign key is typically a primary key of an entity it is related to.

In entity relationship diagrams a weak entity set is indicated by a bold rectangle (the entity) connected by a bold type arrow to a bold diamond (the relationship). It may also be indicated by a double rectangle.

[edit] For Example

Consider a database that records customer orders, where an order is for one or more of the items that the enterprise sells. The database would contain a table identifying customers by a customer number (primary key); another identifying the products that can be sold by a product number (primary key); and it would contain a pair of tables describing orders.

One of the tables could be called Order (except in an SQL database, where ORDER is a reserved word); it would have an order number (primary key) to identify this order uniquely, and would contain a customer number (foreign key) to identify who the products are being sold to, plus other information such as the date and time when the order was placed, how it will be paid for, where it is to be shipped to, and so on.

The other table could be called OrderItem; it would be identified by a compound key consisting of the order number (foreign key) and an item line number, plus the product number (foreign key) that was ordered, the quantity, the price, any discount, any special options, and so on. There may be zero, one or many OrderItem entries corresponding to an Order entry, but no OrderItem entry can exist unless the corresponding Order entry exists. (The zero OrderItem case normally only applies transiently, when the order is first entered and before the first ordered item has been recorded.)

The OrderItem table stores weak entities precisely because an OrderItem has no meaning independent of the Order. Some might argue that an OrderItem does have some meaning on its own; it records that at some time not identified by the record, somebody not identified by the record ordered a certain quantity of a certain product. This information might be of some use on its own, but it is of limited use. For example, as soon as you want to find seasonal or geographical trends in the sales of the item, you need information from the related Order record.

[edit] See also

• Associative entity