One-to-one relationships

- [Instructor] One-to-one relationships … are not nearly as common as one-to-many, … but they do have their place in certain circumstances. … We've already established that they join type is dictated … by the unique constraints. … A one-to-one relationship exists … when both columns in the join … have a unique constraint on them. … This is most commonly seen … when joining primary keys in both tables. … Here's an example of when this might occur. … In this database we have one table … to store common information about some employees, … like their name and position. … This information isn't very sensitive, … and it's made available to everyone in the company. … But the human resources department has a second table … that adds some additional information about each employee, … such as their salary and job rating. … Since both of these tables … use the employee ID as the primary key, … which is by definition unique, … they can be joined in a one-to-one relationship. … Each record in one table …