What is ER diagram?
An ER diagram (presuming it is an Entity-Relationship diagram) is a tool for for depicting presenting, and analyzing data. In its simplest form, it shows entities (things about which you store data, like CUSTOMERS, PRODUCTS, INVOICES, etc.) along with their attributes (the data elements you are storing, like CustomerID, Customer Name, Product Type, Invoice Number, etc.) and how the entities are related to each other. For example, a Customer can have many Invoices and an invoice can contain many Products.
It is a very valuable tool for discovering missing data and for data visualization. It is generally used by data analysts, data base analysts, data administrators, and sometimes business analysts to identify issues with an IT application or to design a new one.
An entity relationship diagram (ERD) shows the relationships of entity sets stored in a database. An entity in this context is a component of data. In other words, ER diagrams illustrate the logical structure of databases.
At first glance an entity relationship diagram looks very much like a flowchart. It is the specialized symbols, and the meanings of those symbols, that make it unique.
How to think while designing ER diagram?
Think of the problem your software is going to solve. And then think of the nouns in the problem definition. This is the easiest way to uncover entities.
For example, in a taxi booking system, entities would be taxi, trip, and customer.
Next, ensure that each proposed entity in your system is independent and cannot be merged into another entity. You don’t want to have five entities where only two will do. For example, a bill might be a potential entity in the example above, until you realise that it is an attribute of a trip.
In this way, come up with as many nouns (things) as you can and then decide if they can be entities in your system, or are merely attributes of entities.
Remember, an entity can be tangible or intangible. At the end, it’s just a noun whose data your software needs to track.
An ER diagram is a means of visualizing how the information a system produces is related. There are five main components of an ERD:
- Entities, which are represented by rectangles. An entity is an object or concept about which you want to store information.A weak entity is an entity that must defined by a foreign key relationship with another entity as it cannot be uniquely identified by its own attributes alone.
- Actions, which are represented by diamond shapes, show how two entities share information in the Insome cases, entities can be self-linked. For example, employees can supervise other employees.
- Attributes, which are represented by ovals. A key attribute is the unique, distinguishing characteristic of the entity. For example, an employee’s social security number might be the employee’s key attribute.
A multivalued attribute can have more than one value. For example, an employee entity can have multiple skill values.A derived attribute is based on another attribute. For example, an employee’s monthly salary is based on the employee’s annual salary.
- Connecting lines, solid lines that connect attributes to show the relationships of entities in the diagram.
- Cardinalityspecifies how many instances of an entity relate to one instance of another entity. Ordinality is also closely linked to cardinality. While cardinality specifies the occurrences of a relationship, ordinality describes the relationship as either mandatory or optional. In other words, cardinality specifies the maximum number of relationships and ordinality specifies the absolute minimum number of relationships.
How to create ER diagram online?
There are many tools which can help you design ER diagrams, try our tool:
- Create database objects like tables, objects, relationships, indexes, quickly and intuitively
- Alter and edit database objects inline in your diagram
- Copy or Move columns across tables
- Zoom in and out of diagrams
- On-the- spot design anywhere on any browser