How to represent foreign key in er diagram?
Suppose I have a 'Transactions' table which has a column 'Customer ID' (Foreign Key) and a Customer Table having 'ID' (Primary key). How do I show the relation between the two tables and showing that the 'Customer ID' is the foreign key of the 'Transactions' Table which is the primary key in the 'Customer' table?
I googled this question and also checked this forum for my query but couldn't find an exact example with a diagram addressing my question.
Please explain to me, if possible, with a diagram.
The answer to your question - how to represent foreign key in er diagram is -
ER Diagrams were originally used only to represent the ER model. The ER model does not use foreign keys to represent relationships. It uses lines between boxes. The lines have some kind of indicator for cardinality at either end or both ends. Sometimes, a relationship will be indicated separately by a diamond.
Today, more than half of the ER diagrams floating around are really diagrams of a relational model, and not of an ER model. A relational model has the foreign keys included in the tables, and these serve to implement the relationships which the ER model identifies. And a relational model will have an extra table, often called a "junction table" between two entity tables that are linked by a many-to-many relationship. This junction table contains two or more foreign keys.
There are many ways to represent a relational model. Perhaps the simplest is the "Relationship Diagram" that MS Access can produce from a completed database. This will be fairly complete, if the database builder has identified the foreign keys. There are many tools that are more sophisticated than MS Access for making diagrams on a larger scale. Some of these are used before building the database. Some are used after.
Answer (1)
In an Entity-Relationship (ER) diagram, a foreign key is typically represented by a connecting line between two entities, indicating a relationship between them. Here's how you can represent a foreign key in an ER diagram:
Identify Entities: Begin by identifying the entities in your database schema. Entities represent real-world objects or concepts, such as "Customer," "Order," or "Product."
Define Relationships: Determine the relationships between entities. A foreign key represents a relationship between two entities, where one entity references the primary key of another entity.
Draw Connecting Lines: To represent a foreign key relationship, draw a connecting line between the entity containing the foreign key attribute and the entity containing the referenced primary key. The line should connect the foreign key attribute in one entity to the primary key attribute in the other entity.
Add Cardinality and Optionality: Optionally, you can add cardinality and optionality indicators to the connecting lines to specify the nature of the relationship. Cardinality indicates how many instances of one entity are associated with instances of another entity, while optionality indicates whether the relationship is mandatory or optional.
For example, a "1" on the line indicates a one-to-many relationship, where each instance of one entity can be associated with multiple instances of another entity.
Similarly, a "0..1" indicates an optional relationship, where an instance of one entity may or may not be associated with an instance of another entity.
Here's a simple example:
Entity A (with primary key attribute A_ID)
Entity B (with foreign key attribute A_ID referencing Entity A's primary key)
In the ER diagram, you would draw a line connecting the A_ID attribute in Entity B to the A_ID attribute in Entity A, indicating the foreign key relationship between them.
Remember to follow any conventions or guidelines specific to the notation you're using for ER diagrams, as different notations may have slightly different representations for foreign keys.