In an Entity-Relationship Diagram (ERD), a one-to-one relationship represents a scenario where each entity in one table is associated with exactly one entity in another table, and vice versa. Here are the primary characteristics:
Uniqueness: Each instance of an entity in one table corresponds to one and only one instance in the related table. This is often used when you want to split data into two tables for logical or performance reasons.
Primary Key Usage: A one-to-one relationship often involves sharing the primary key between tables. One table will have a primary key, and the other table might use it as a primary key as well as a foreign key, ensuring each entity pair is unique.
Data Segmentation: This relationship is commonly used to separate data that may be sensitive, infrequently accessed, or logically distinct from the main entity. For instance, you might separate personal information from general information.
Optionality: Sometimes, a one-to-one relationship is optional, meaning that not every instance in the primary table has a corresponding entry in the secondary table. In such cases, the foreign key in the secondary table may be nullable.
Rare in Practice: True one-to-one relationships are relatively rare in database design, as they can often be merged into a single table unless there is a compelling reason to separate the data.
Examples include splitting user profile data into a separate table for privacy (e.g., a table for basic account information and another for detailed personal info that is only occasionally accessed).
Further Explanation of (1:1) Relationships
A one-to-one (1:1) relationship exists when zero or one instance of entity A can be associated with zero or one instance of entity B, and zero or one instance of entity B can be associated with zero or one instance of entity A.
To illustrate such a relationship graphically, think of entities A and B in terms of generic tables, where generic tables are not populated with data.A one-to-one relationship exists when one record in Table A can be associated with one record in Table B, and one record in Table B can be associated with one record in Table A. The following diagrams illustrate a 1:1 relationship between Table A (entity A) and Table B (entity B) from the viewpoint first of Table A and then of Table B.
1) 1:1 relationship between Table A (entity A) and Table B (entity B) from the viewpoint of Table A. Note that one record in Table A relates to one record in Table B (A:B = 1:1)
2) The same relationship between Tables A and B appears this way from the viewpoint of Table B.
Note that one record in Table B relates to one record in Table A (B:A = 1:1)
Mandatory Relationship:When transforming a one-to-one relationship, you create a foreign key and a unique key and all columns of this foreign key are also part of a unique key. If the relationship is mandatory on one side, the foreign key is created at the corresponding table. If the relationship is mandatory on both sides or optional on both sides, you can choose on which table you want to create the foreign key. There is no absolute rule for deciding on which side to implement it.
Optional Relationship:If the relationship is optional on both sides you may decide to implement the foreign key in the table with fewer numbers of rows, as this would save space. If the relationship is mandatory at both ends, we are facing the same RDBMS limitation you saw earlier. Therefore, you need to write code to check the mandatory one at the other side, just as you did to implement m:1 relationships that are mandatory at the one end.
Real-world use cases for "one-to-one" Relationships
There are real-world use cases for "one-to-one" relationships in data modeling, although they are less common than other types like one-to-many or many-to-many. Here are a few examples:
User Profile and Authentication Data: In a system where user profile information (like name, contact info, and address) is stored separately from authentication data (like password and security settings), you might use a one-to-one relationship between the UserProfile and UserAuth tables. Each user has one profile and one set of authentication details, but keeping them separate can improve security and modularity.
Employee and Company Car Assignment: In an organization where each employee is assigned a specific company car, a one-to-one relationship might exist between the Employee and Car tables. Each employee has one assigned car, and each car is assigned to one employee.
Primary and Secondary Address: In some cases, a primary address might be stored in a main User table, while a secondary address is stored in a related SecondaryAddress table. This is especially useful if you want to store secondary addresses only for users who have them, allowing you to save space by not storing null values in a single address field.
Sensitive Data Separation: In healthcare or finance, sensitive data (such as social security numbers or medical records) may be stored in a separate, highly secured table linked by a one-to-one relationship with the main entity. This separation helps with security compliance, as only authorized personnel can access this specific data.
Configuration Settings for Specific Users: If certain users have specific configuration settings or preferences, a one-to-one relationship can be used to link the main User table with a UserSettings table that holds this information. This approach avoids adding many optional columns to the main table and keeps settings modular.
These examples illustrate how one-to-one relationships can help with modularity, security, and efficient data storage in specific situations.
The connectivity of a relationship describes a constraint on the connection of the associated entity occurrences in the relationship. Values for connectivity are either
one or
many.
For a relationship between the entities Department and Employee, a connectivity of one for Department and many for Employee means that there is at most one entity occurrence of Department associated with many occurrences of Employee. The actual count of elements associated with the connectivity is called the cardinality of the relationship connectivity; it is used much less frequently than the connectivity constraint because the actual values are usually variable across instances of relationships. Note that there are no standard terms for the connectivity concept, so the reader is admonished to consider the definition of these terms carefully when using a particular database design methodology.
The next lesson explores one-to-one relationships further.
