The term one-to-one relationships refers to relationships of two items in which one can only belong with the other. These relationships can be referred to in a mathematical sense, in which there are equal numbers of items, or when creating a database when one row directly corresponds to another row. Discover more about one-to-one relationships that you might see in your everyday life.
You can find one-to-one (or 1:1) relationships everywhere. They describe a relationship in which one item can only be paired with another item. But in order to be a one-to-one relationship, you must be able to flip the relationship so that it’s true both ways.
For example, one student has one teacher. But that teacher probably has a whole classroom of students, making that relationship one-to-many instead of one-to-one. It would be more accurate to say that each student has one backpack, and that each backpack belongs to one student.
Look around your house. How many one-to-one relationships do you see? Here are some examples of one-to-one relationships in the home:
- One family lives in one house, and the house contains one family.
- One person has one passport, and the passport can only be used by one person.
- One person has one ID number, and the ID number is unique to one person.
- A person owns one dog, and the dog is owned by one person.
- In monogamous relationships, one person has one partner, who is only partnered with that person.
- One person owns one car, and the car is owned by one person.
- One child sleeps in one bed, and the bed is used by one child.
Some examples that are not one-to-one relationships (and are therefore one-to-many relationships) are kitchen appliances. You may only be able to bake a cake in an oven, for example, but the oven can be used for many different foods. Purchasing an appliance that only has one purpose wouldn’t be the most versatile use of your money!
A popular place to hear the phrase “one-to-one” is in the classroom. Many school districts want their students to each have an electronic device, such as a tablet or laptop. This describes a “1:1 classroom.”
Other examples of one-to-one relationships in education are:
- Each student must complete one worksheet (and the worksheet can only be completed by one student).
- One student can check out a book from the library (and the library book can be checked out by one student at a time).
- Each student gets to buy one lunch from the cafeteria (and each lunch can only be purchased by one student).
- Each small group in the class must complete a project about one country (and that country can only be researched by one small group in the class).
- Each student gets one desk (which can only be sat in by one student).
Some of these examples can become one-to-many examples when you think of multiple classes. For example, one student may occupy one desk in their first period class, but in the second period, another student will sit there. Each student has one desk, but over the course of the day, the desk will have many students. However, the one-to-one relationship between students and desks stays consistent per period.
Businesses know the value of one-to-one relationships. Exclusive agreements and connections often help businesses set themselves apart from their competition. Employees in a company are also individuals with specialized skills as well.
Here are some examples of true one-to-one relationships in business:
- An advertising firm works with only one company’s account, and that company uses the firm for all of their advertising needs.
- A company creates only one product, and that product is only made by that company.
- A company offers only one service, and that service is only offered by that company.
- A CEO hires only one personal assistant, and that assistant only works with the CEO.
- One employee completes one specialized task, and that task is only completed by that employee.
- Each employee has their own office, and each office is only occupied by one employee.
- Each employee has a specific phone extension, which can only reach one employee.
Companies that thrive in the 21st century know that they must have a close relationship with their customers. They want customers to think that this is a one-to-one relationship. However, if a company is successful, they have this relationship with many customers – making it a one-to-many relationship in reality.
The natural world has many one-to-one relationships. Because of their need for biological diversity, there are unique relationships in plant life and animal life alike. Here are some examples of one-to-one relationships in nature:
- Leopards have individual spot patterns, and each spot pattern belongs to only one leopard.
- Penguins mate with one other penguin, who only mates with that penguin.
- Mammals who give birth to one baby raise that baby, who are only raised by that mother.
- Humans have unique fingerprints, which only belong to that human.
- A bear may live in a cave that is only home to that bear.
- A hermit crab can only live in one shell at a time, and that shell can only house one hermit crab at a time.
It’s tempting to say that a plant or animal has a one-to-one relationship with their habitat, especially if that plant or animal is only found in a specific part of the world. However, every habitat on Earth contains more than one type of plant or animal, making the relationship of a plant or animal a one-to-many relationship.
Some forms of one-to-one relationships are present in your everyday life, but they’re not as obvious as the examples above. Databases, marketing, and mathematics all use one-to-one relationships in their basic functions.
Relational databases have three types of relationships between data entries: one-to-one, one-to-many, and many-to-many. A straightforward relational database links one item to another, and the second item to the first item.
For example, think of a list of names and phone numbers. Each name corresponds to only one phone number, and each phone number links only to the correct name in a one-to-one relationship. A database that links names to multiple points of information, such as phone numbers, addresses, social security numbers, describes one-to-many relationships. Many-to-many relationships list many pieces of data (multiple people) to many other pieces of data (their attributes and information).
Companies use one-to-one marketing to directly target potential customers and users. They collect information about customers based on previous interactions and tailor advertisements specifically for them. Think about the last time you ordered something online, and then clicked to another browser just to see an ad for the very same item! That’s one-to-one marketing.
One-to-one relationships in math are known as cardinality. The ability of a student to identify the number one as corresponding to one item, the number two as corresponding to two items, the number three as corresponding to three items is an example of one-to-one relationships known as "one-to-one correspondence."
In any context, one-to-one relationships keep us connected to each other. This interdependence is the basis for life all over the world. Learn more about the connections between animals and their habitats with these examples of symbiotic relationships.