Business analysts and database engineers use ER diagrams as data modeling tools to assess the scope of the databases their organizations need, then plan out how the data will be stored. 

ERDs inform the software engineering portion of a database project by laying out the requirements for the information systems architecture and database structure. In the three-schema approach to software engineering for database management systems (DBMS), the ERD is the conceptual tier. 

Data integration is a complex data engineering process consisting of many moving parts. An ERD can help data engineers conceptualize the overall system and reduce the potential for errors. 

Comparing existing databases to an ERD can reveal database design missteps that might be causing problems. Complex databases with numerous tables require extensive SQL knowledge for the debugging process. An ERD summarizes the database so engineers can quickly identify potential errors. 

When undertaking business process reengineering projects, it can be helpful to obtain a bird’s-eye view of all the data within an organization’s information systems. ERDs are used to draft newer, more efficient data architecture solutions that facilitate the other stages of the BPR process.

Entity types are a category of entities. If entities are similar to nouns, then entity types are noun categories: foods, sports and countries. The individual entities within an entity type are known as instances. Within the entity type, vegetables might be the instances broccoli, carrot and asparagus.

Entities are classified as either strong or weak. Strong entities contain enough identifying information in their attributes to not need further clarification. Meanwhile, weak entities exist only as an outcome or consequence of another entity. The strong entity associated with a given weak entity is known as its parent or owner entity.

Consider a database modeling customer order in an e-commerce business. Each order is a strong entity because it can be defined as a unique instance based on the purchaser, time and date. However, the line items within each order are weak entities. They only have meaning within the context of their respective orders. This reliance is known as existence dependency or participation constraint.

Strong entities are shown as solid rectangles, while ERDs represent weak entities as a double rectangle.

An associative entity links the instances between two entity sets and has its own attributes that provide more information about that relationship. In an ERD used by a university, the entity sets students and professors to have many connections to each other. The associative entity bridging the two would show which students are taking courses taught by which professors.

Relational databases use associative entities to inform junction tables, which combine fields from multiple other database tables. In ER diagrams, associative entities are depicted as a diamond within a rectangle.

• Simple attributes cannot be simplified or split up into further attributes. A ZIP code is an example of a simple attribute.

• Composite attributes are compiled from other attributes, which might or might not be simple. An address is a composite attribute containing a street number, street name, ZIP code, city and other identifying information.

• Derived attributes are calculated based on other attributes. An employee’s paycheck value is derived from their hours worked, the duration of the pay period, and their wage. ERDs depict derived attributes as dashed ovals.

• Multivalue attributes can have more than one value per record, while a single-value attribute cannot.

Entity keys are the attributes that uniquely define each entity in a data set. Any attribute can be designated as a key, provided that it fills this role. For example, in a people entity set, an appropriate key attribute might be a national ID number. Conversely, surnames would not work as a key attribute in this context since more than one person can share the same surname. 

• Super key: One or more attributes that can uniquely define an entity within an entity set.

• Candidate key: The simplest possible super key—no attributes within a candidate key can be a super key in itself. Candidate keys might consist of one or more attributes if each attribute is not a super key. 

• Primary key: The candidate key is chosen to uniquely define an entity set. Since the primary key is what sets each entity apart, no two entries in a database might share the same primary key value. In an ER diagram, each entity’s primary key will be underlined. Any entity containing a primary key is considered a strong entity. 

• Foreign key: An attribute that identifies one entity’s relationship to another. Weak entities rely on foreign keys to define them as strong entities. For example, the weak entity bank account would need a foreign key linking it to the relevant bank. 

Cardinality is the quality of a relationship that defines the number of instances in one entity that relate to the instances of another.

• One-to-one relationships (1:1) indicate that a record within one entity might only be referenced by one record in the other entity. The relationship between the entities' universities and presidents is a one-to-one relationship because every university has only one president. Conversely, each president is responsible for exactly one university.

• One-to-many relationships (1:M) depict situations in which each record within one entity relates to multiple records in another entity. There is a one-to-many relationship between the entities universities and departments. A university might have multiple departments, but each department is part of just one university.

• Many-to-many relationships (M:M) show that one or more records within both entities can be connected. The entities students and professors have a many-to-many relationship because just as one professor teaches a class with many students, each student might also enroll in classes with other professors.

ERDs represent cardinality through variations in the connecting lines between entities. The way cardinality is shown depends on the style of ERD used.