1. COMP 430 Intro. to Database Systems
Joins

2. A bit of review
Slides copied from earlier in course

3. Joining tables SELECT * FROM Product, Company
p_name
price
manufacturer
Gizmo
19.99
GizmoWorks
Powergizmo
39.99
Widget
WidgetsRUs
HyperWidget
203.99
Hyper
c_name
address
city
state
GizmoWorks
123 Gizmo St.
Houston TX
WidgetsRUs
20 Main St.
New York NY
Hyper
1 Mission Dr.
San Francisco CA
SELECT *
FROM Product, Company
WHERE manufacturer = c_name;
Join condition
p_name
price
manufacturer
address
city
state
Gizmo
19.99
GizmoWorks
123 Gizmo St.
Houston TX
Powergizmo
39.99
Widget
WidgetsRUs
20 Main St.
New York NY
HyperWidget
203.99
Hyper
1 Mission Dr.
San Francisco CA

4. Joining tables
Product
Company
p_name
price
manufacturer
Gizmo
19.99
GizmoWorks
Powergizmo
39.99
Widget
WidgetsRUs
HyperWidget
203.99
Hyper
c_name
address
city
state
GizmoWorks
123 Gizmo St.
Houston TX
WidgetsRUs
20 Main St.
New York NY
Hyper
1 Mission Dr.
San Francisco CA
This is simplest & most common kind of join – an inner join. See other kinds later.
p_name
price
manufacturer
address
city
state
Gizmo
19.99
GizmoWorks
123 Gizmo St.
Houston TX
Powergizmo
39.99
Widget
WidgetsRUs
20 Main St.
New York NY
HyperWidget
203.99
Hyper
1 Mission Dr.
San Francisco CA

5. Joins – forgetting the join condition
SELECT *
FROM Product, Company;
p_name
price
manufacturer
c_name
address
city
state
Gizmo
19.99
GizmoWorks
123 Gizmo St.
Houston TX
WidgetsRUs
20 Main St.
New York NY
Hyper
1 Mission Dr.
San Francisco CA
Powergizmo
39.99
Widget …
All combinations of records! – Cross-product of tables.

6. Selection & Projection
SELECT p_name
FROM Product, Company
WHERE manufacturer = c_name AND
state = ‘TX’;
Join condition
Filter condition
p_name
price
manufacturer
address
city
state
Gizmo
19.99
GizmoWorks
123 Gizmo St.
Houston TX
Powergizmo
39.99
Selection
p_name
Gizmo
Powergizmo
Projection

7. So what else is there?

8. Inner join syntax Old style: Current, preferred style: SELECT *
FROM Product, Company
WHERE manufacturer = c_name;
SELECT *
FROM Product
INNER JOIN Company ON manufacturer = c_name;
SELECT *
FROM Company
INNER JOIN Product ON manufacturer = c_name;
Shorter
Easier to find tables
Equal status for tables – symmetry
Can’t forget join condition
Join condition separated from filter conditions
Consistent with other joins

9. Two versions equivalent
SELECT *
FROM Product, Company
WHERE manufacturer = c_name;
SELECT *
FROM Product
INNER JOIN Company ON manufacturer = c_name;
Product (p_name, price, manufacturer)
Company (c_name, address, city, state)
Result (p_name, price, manufacturer, c_name, address, city, state)
Previously lied a little. Both fields present.

10. Inner join – special case
Both tables use the same attribute name.
Product (p_name, price, c_name)
Company (c_name, address, city, state)
SELECT *
FROM Product
INNER JOIN Company ON Product.c_name = Company.c_name;
Join condition is simple equality. Equi-join
Result (p_name, price, Product.c_name, Company.c_name, address, city, state)
SELECT *
FROM Product
INNER JOIN Company USING (c_name);
Can list multiple attributes.
Result (p_name, price, c_name, address, city, state)

11. Inner join – another special case
Product (p_name, price, c_name)
Company (c_name, address, city, state)
Considered dangerous, since it constraint is implicit, and attribute name sets can change.
Don’t use.
SELECT *
FROM Product
INNER JOIN Company USING (c_name);
Result (p_name, price, c_name, address, city, state)
SELECT *
FROM Product
NATURAL INNER JOIN Company;
Assumes equality of same-named attributes. Equi-join
Result (p_name, price, c_name, address, city, state)

12. What about dangling tuples?
Company
Product
c_name
state
GizmoWorks TX
WidgetsRUs NY
Hyper CA
NewCo
p_name
manufacturer
Gizmo
GizmoWorks
Powergizmo
Widget
WidgetsRUs
HyperWidget
Hyper
c_name
state
p_name
manufacturer
GizmoWorks TX
Gizmo
Powergizmo
WidgetsRUs NY
Widget
Hyper CA
HyperWidget
NewCo is forgotten in join, since it has no products.
SELECT *
FROM Company
INNER JOIN Product ON manufacturer = c_name;

13. Left outer join
Company
Product
c_name
state
GizmoWorks TX
WidgetsRUs NY
Hyper CA
NewCo
p_name
manufacturer
Gizmo
GizmoWorks
Powergizmo
Widget
WidgetsRUs
HyperWidget
Hyper
c_name
state
p_name
manufacturer
GizmoWorks TX
Gizmo
Powergizmo
WidgetsRUs NY
Widget
Hyper CA
HyperWidget
NewCo
NULL
“LEFT” because result includes dangling tuples from the left table..
Original tables have no data.
SELECT *
FROM Company
LEFT OUTER JOIN Product ON manufacturer = c_name;

14. Right outer join
Company
Product
c_name
state
GizmoWorks TX
WidgetsRUs NY
Hyper CA
p_name
manufacturer
Gizmo
GizmoWorks
Powergizmo
Widget
WidgetsRUs
HyperWidget
Hyper
c_name
state
p_name
manufacturer
GizmoWorks TX
Gizmo
Powergizmo
WidgetsRUs NY
Widget
Hyper CA
HyperWidget
Useless in this example. Referential integrity guarantees Product has no dangling tuples.
SELECT *
FROM Company
RIGHT OUTER JOIN Product ON manufacturer = c_name;

15. Assume no referential integrity now.
Right outer join
Company
Product
c_name
state
GizmoWorks TX
WidgetsRUs NY
Hyper CA
p_name
manufacturer
Gizmo
GizmoWorks
Powergizmo
Widget
WidgetsRUs
HyperWidget
Hyper
NewThing
NULL
c_name
state
p_name
manufacturer
GizmoWorks TX
Gizmo
Powergizmo
WidgetsRUs NY
Widget
Hyper CA
HyperWidget
NULL
NewThing
Assume no referential integrity now.
SELECT *
FROM Company
RIGHT OUTER JOIN Product ON manufacturer = c_name;

16. Includes dangling tuples from both sides.
Full outer join
Company
Product
c_name
state
GizmoWorks TX
WidgetsRUs NY
Hyper CA
NewCo
p_name
manufacturer
Gizmo
GizmoWorks
Powergizmo
Widget
WidgetsRUs
HyperWidget
Hyper
NewThing
NULL
c_name
state
p_name
manufacturer
GizmoWorks TX
Gizmo
Powergizmo
WidgetsRUs NY
Widget
Hyper CA
HyperWidget
NewCo
NULL
NewThing
Includes dangling tuples from both sides.
SELECT *
FROM Company
FULL OUTER JOIN Product ON manufacturer = c_name;

17. Cross join SELECT * FROM Product
CROSS JOIN Company;
CROSS JOIN doesn’t have join condition.
p_name
price
manufacturer
c_name
address
city
state
Gizmo
19.99
GizmoWorks
123 Gizmo St.
Houston TX
WidgetsRUs
20 Main St.
New York NY
Hyper
1 Mission Dr.
San Francisco CA
Powergizmo
39.99
Widget …
All combinations of records! – Cross-product of tables.

18. A few notes
There are various short-hands, e.g., JOIN = INNER JOIN, LEFT JOIN = LEFT OUTER JOIN.
Outer joins also allow NATURAL, USING.
Left outer join of tables A,B = Right outer join of tables B,A.
SQLite doesn’t implement Right & Full outer joins.
These “joins” describe semantics. Other “joins” describe implementation.

19. Visual summary Of inner & outer joins
Examples adapted from Coding Horror.

20. Records matching both A and B.id
data 1 A 2 B 3 C 4 D id
A.data
B.data 2 B W 4 D X B id
data 2 W 4 X 6 Y 8 Z
SELECT *
FROM A
INNER JOIN B ON A.id = B.id;
Records matching both A and B.

21. FULL OUTER JOIN B ON A.id = B.id; Records matching A or B.
data 1 A 2 B 3 C 4 D id
A.data
B.data 1 A 2 B W 3 C 4 D X 6 Y 8 Z B id
data 2 W 4 X 6 Y 8 Z
SELECT *
FROM A
FULL OUTER JOIN B ON A.id = B.id;
Records matching A or B.

22. Records matching either A or B, but not both.id
data 1 A 2 B 3 C 4 D id
A.data
B.data 1 A 3 C 6 Y 8 Z B id
data 2 W 4 X 6 Y 8 Z
SELECT *
FROM A
FULL OUTER JOIN B ON A.id = B.id
WHERE A.data IS NULL OR B.data IS NULL;
Records matching either A or B, but not both.

23. LEFT OUTER JOIN B ON A.id = B.id; Records matching A.
data 1 A 2 B 3 C 4 D id
A.data
B.data 1 A 2 B W 3 C 4 D X B id
data 2 W 4 X 6 Y 8 Z
SELECT *
FROM A
LEFT OUTER JOIN B ON A.id = B.id;
Records matching A.

24. Records matching only A.id
data 1 A 2 B 3 C 4 D id
A.data
B.data 1 A 3 C B id
data 2 W 4 X 6 Y 8 Z
SELECT *
FROM A
LEFT OUTER JOIN B ON A.id = B.id
WHERE B.data IS NULL;
Records matching only A.