Sometimes you need to display data from more than one table. To do this, you use SELECT statements. The FROM clause of your query contains the names of the tables from which you are retrieving data. Because information comes from more than one table, this is called a JOIN between the tables involved.
Note: Without the proper join, the result will be the Cartesian product of all records in both tables.
For example, in the EMPLOYEES table, the DEPARTMENT_ID column represents the department number for an employee. In the DEPARTMENTS table, there is a DEPARTMENT_ID column as well as a DEPARTMENT_NAME column. You can join the EMPLOYEES and DEPARTMENTS tables by using the DEPARTMENT_ID column to produce a report that shows the employees' names and the name of the department they belong to, rather than just the department id.