In a Database Management System what are joins?

A Database Management System or DBMS is compromised of many related tables of data. Tables are simply two dimentional (rows and columns) of data, similar to a spreadsheet. Joins are used to combine tables of data based on related columns within those tables. By using JOINS, a database becomes multi-dementional. Joins are simply a structured way to combine related data in different tables within the database system.

Example an employee table might contain the following fields:

EmployeeID (the Primary Key)

LastName,

FirstName,

MiddleName,

HireDate,

DepartmentID (a foreign key to the Department table)

Manager_EmployeeID (a foreign key relating back to a different employee record in the employee table)

Department Table:

DepartmentID,

DepartmentName,

DepartmentManager_EmployeeID (Foriegn Key relating to the employee table)

In the Employee sample table, the EmployeeID serves as the primary key, which uniquely identifies the entire row of data. Joins for the Employee table would typically be done on related tables, such as the Department table via the DepartmentID, or even back on the Employee table itself using the Manager_EmployeeID, relating the EmployeeID to the Manager_EmployeeID. If the EmployeeID is in another table as a foreign key then joins can also be done to those tables as well.