A JOIN operation is performed when more than one table is specified in the FROM clause.  You would join two tables if you need information from both. You must specify the JOIN condition explicitly in SQL.  This includes naming the columns in common and the comparison operator.

Example 1

Find the name and courses that each faculty member teaches.
SELECT FACULTY.FACNAME, COURSENUM FROM FACULTY, CLASS WHERE FACULTY.FACID = CLASS.FACID;

FACULTY.FACNAME	COURSENUM
Adams	ART103A
Tanaka	CIS201A
Byrne	MTH101B
Smith	HST205A
Byrne	MTH103C
Tanaka	CIS203A

When both tables have an attribute name in common, you must specify which version of the attribute that you are referring to by preceding the attribute name with the table name and a period.  (e.g., table‑name.col‑name).  This is called “qualification”.

It is sometimes more convenient to use an “alias” (an alternative name) for each table.  SQL specifies alias names in the FROM clause immediately following the actual table.  Once defined, you can use the alias anywhere in the SELECT where you would normally use the table name.

Example 2

Find the course number and the major of all students taught by the faculty member with ID number ‘F110’. (3 table JOIN)
SELECT ENROLL.COURSENUM, LNAME, MAJOR FROM CLASS , ENROLL, STUDENT WHERE FACID = ‘F110’ AND CLASS.COURSENUM = ENROLL.COURSENUM AND ENROLL.STUID = STUDENT.STUID;

ENROLL.COURSENUM	LNAME	MAJOR
MTH101B	Rivera	CIS
MTH103C	Burns	ART
MTH103C	Chin	Math

Using aliases, this would be:
SELECT E.COURSENUM, LNAME, MAJOR FROM CLASS C, ENROLL E, STUDENT S WHERE FACID = ‘F110’ AND C.COURSENUM = E.COURSENUM AND E.STUID = S.STUID;