Self joins

- [Instructor] Relationships are almost always made … between two different data tables … but it is possible for a column and a table … to relate to another column in the same table. … This is called a self join. … A self join, also known as … a self-referencing relationship or a recursive relationship … follows all of the same rules … as the relationships created between two tables. … They can be one-to-one, one-to-many, … or many-to-many relationships. … The same unique constraints still apply … and you still you need to consider cardinality … when determining the type of join. … So where might a self join be useful? … They're used to model hierarchies within the same class. … For instance, consider this table of employees. … Each employee has an ID … and the table also stores the ID of … the employee's direct supervisor. … If you mentally connect the supervisor ID column … back to the employee ID column, … you can see that Marisol is supervisor … to both Josiah and Severino … and Severino is Shea's supervisor. …