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;