COMP 430 Intro. to Database Systems Joins

A bit of review Slides copied from earlier in course

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

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

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.

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

So what else is there?

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

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.

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)

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)

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;

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;

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;

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;

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;

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.

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.

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

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.

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.

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.

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.

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.