Data Modeling | Identifying and non-identifying relationships
Expense aside, there is no comparison between the productivity possible in the For some of you, I'm sure the term identifying relationship brings back memories Instead, you're dealing with the relationships that are defined by foreign keys. Non-identifying relations are displayed as dotted lines with black circle on the child end. If this relation is an Optional Non-Identifying Relationship (child foreign . The one-to-many non-identifying relationship tool An identifying relationship is one where the child table cannot be uniquely identified without its parent.
Learning MySQL by Hugh E. Williams, Saied M.M. Tahaghoghi
A student and a course are entities, while an enrollment is a relationship between a student and a course. Similarly, an inventory and sales database might store information about products, customers, and sales. A product and a customer are entities, while a sale is a relationship between a customer and a product.
A popular approach to conceptual design uses the Entity Relationship ER model, which helps transform the requirements into a formal description of the entities and relationships that appear in the database. In the ER diagram, an entity set is represented by a rectangle containing the entity name. An entity set is represented by a named rectangle We typically use the database to store certain characteristics, or attributes, of the entities.
In a sales database, we could store the name, email address, postal address, and telephone number for each customer. Attributes describe the entity they belong to. An attribute may be formed from smaller parts; for example, a postal address is composed of a street number, city, ZIP code, and country. Some attributes can have multiple values for a given entity. For example, a customer could provide several telephone numbers, so the telephone number attribute is multivalued.
Attributes help distinguish one entity from other entities of the same type. We could use the name attribute to distinguish between customers, but this could be an inadequate solution because several customers could have identical names. To be able to tell them apart, we need an attribute or a minimal combination of attributes guaranteed to be unique to each individual customer.
The identifying attribute or attributes form a key. In our example, we can assume that no two customers have the same email address, so the email address can be the key. However, we need to think carefully about the implications of our choices.
For example, if we decide to identify customers by their email address, it would be hard to allow a customer to have multiple email addresses. Any applications we build to use this database might treat each email address as a separate person, and it might be hard to adapt everything to allow people to have multiple email addresses.
The Entity Relationship Model - Learning MySQL [Book]
Clearly, there may be several possible keys that could be used to identify an entity; we choose one of the alternative, or candidate, keys to be our main, or primary, key. You usually make this choice based on how confident you are that the attribute will be non-empty and unique for each individual entity, and on how small the key is shorter keys are faster to maintain and use. Attributes comprising the primary key are shown underlined.
The parts of any composite attributes are drawn connected to the oval of the composite attribute, and multivalued attributes are shown as double-lined ovals.
Similarly, a product price could be a positive rational number. Attributes can be empty; for example, some customers may not provide their telephone numbers. You should think carefully when classifying an attribute as multivalued: The sales database requirements may specify that a product has a name and a price.Entity Relationship Diagram (ERD) Tutorial - Part 1
To distinguish between products, we can assign a unique product ID number to each item we stock; this would be the primary key. Each product entity would have name, price, and product ID attributes. The ER diagram representation of the product entity Representing Relationships Entities can participate in relationships with other entities.
For example, a customer can buy a product, a student can take a course, an artist can record an album, and so on. Like entities, relationships can have attributes: Our database could then record each sale and tell us, for example, that at 3: For example, each customer can buy any number of products, and each product can be bought by any number of customers. This is known as a many-to-many relationship. We can also have one-to-many relationships.
For example, one person can have several credit cards, but each credit card belongs to just one person. Looking at it the other way, a one-to-many relationship becomes a many-to-one relationship; for example, many credit cards belong to a single person.
Finally, the serial number on a car engine is an example of a one-to-one relationship; each engine has just one serial number, and each serial number belongs to just one engine. We often use the shorthand terms 1: N for one-to-one, one-to-many, and many-to-many relationships, respectively.
The number of entities on either side of a relationship the cardinality of the relationship define the key constraints of the relationship.
There are many relationships that may at first seem to be one-to-one, but turn out to be more complex. For example, people sometimes change their names; in some applications, such as police databases, this is of particular interest, and so it may be necessary to model a many-to-many relationship between a person entity and a name entity. Redesigning a database can be time-consuming if you assume a relationship is simpler than it really is.
In an ER diagram, we represent a relationship set with a named diamond. The cardinality of the relationship is often indicated alongside the relationship diamond; this is the style we use in this book. Complete subtype relationship, when all categories are known. Incomplete subtype relationship, when all categories may not be known. The standard example of a complete subtype relationship is the party entity.
When sub-type relationships are rendered in a database, the super-type becomes what is referred to as a base table. The sub-types are considered derived tables, which correspond to weak entities. Referential integrity is enforced via cascading updates and deletes. Example[ edit ] 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 Orders and 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 both the order number foreign key and an item line number; with other non-primary key attributes such as 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.