Query Many to Many Mapping

Question

Thanks for looking at my question.
How do I write a query to find values in a many to many relationship?
My tables are normalized similarly to this:
http://download.oracle.com/docs/cd/B25221_05/web.1013/b13593/relmapun008.htm#CHDFDIHC
How can i write a query for 'Jane Smith' that would return all of the projects ('Java Developer', 'Magazine') he is working on? Or, vise versa a query for a project that would return all of the employees mapped to it by the relationship table?
Edited by: 869021 on Jun 28, 2011 4:00 PM

Answer 1

Thanks for the reply. I think it is a little beyond me. I am four weeks into a beginning Data modeling class. I will definitely come back and look at it again later.
I came up with this solution. Just nesting selects until I can query across the relationships.
DROP TABLE Stock;
DROP TABLE Model;
DROP TABLE Dealer;
CREATE TABLE Dealer(
     DealerID               NUMBER,
     DealerName          VARCHAR2(20),
     CONSTRAINT pk_Dealership PRIMARY KEY (DealerID)
CREATE TABLE Model(
     ModelNumber     NUMBER,
     ModelName          VARCHAR2(20),
     CONSTRAINT pk_Model PRIMARY KEY (ModelNumber)
CREATE TABLE Stock(
     StockKey               NUMBER,
     ModelNumber     NUMBER,
     DealerID               NUMBER,
     CONSTRAINT pk_Stock PRIMARY KEY (StockKey),
     CONSTRAINT fk_ModelNumber FOREIGN KEY (ModelNumber) REFERENCES Model (ModelNumber),
     CONSTRAINT fk_DealerID FOREIGN KEY (DealerID) REFERENCES Dealer (DealerID)
INSERT INTO Dealer VALUES(1, 'Midnight Movers');
INSERT INTO Dealer VALUES(2, 'Bubbas Imports');
INSERT INTO Dealer VALUES(3, 'Import Motors');
INSERT INTO Model VALUES(1, 'Buick Skylark');
INSERT INTO Model VALUES(2, 'H2 Hummer');
INSERT INTO Model VALUES(3, 'Chevy Suburban');
INSERT INTO Model VALUES(4, 'BMW Z3');
INSERT INTO Model VALUES(5, 'BMW 328i');
INSERT INTO Model VALUES(6, 'Jeep Wrangler');
INSERT INTO Model VALUES(7, 'Ford Focus');
INSERT INTO Model VALUES(8, 'Range Rover');
INSERT INTO Model VALUES(9, 'Toyota Supera');
INSERT INTO Model VALUES(10, 'Ferrari Testarosa');
INSERT INTO Model VALUES(11, 'Jaguar X12');
INSERT INTO Model VALUES(12, 'Ford Fairmont');
INSERT INTO Stock VALUES(1, 1, 1);
INSERT INTO Stock VALUES(2, 2, 1);
INSERT INTO Stock VALUES(3, 3, 1);
INSERT INTO Stock VALUES(4, 4, 1);
INSERT INTO Stock VALUES(5, 5, 1);
INSERT INTO Stock VALUES(6, 6, 1);
INSERT INTO Stock VALUES(7, 1, 2);
INSERT INTO Stock VALUES(8, 2, 2);
INSERT INTO Stock VALUES(9, 1, 2);
INSERT INTO Stock VALUES(10, 4, 2);
INSERT INTO Stock VALUES(11, 5, 2);
INSERT INTO Stock VALUES(12, 6, 2);
INSERT INTO Stock VALUES(13, 7, 3);
INSERT INTO Stock VALUES(14, 8, 3);
INSERT INTO Stock VALUES(15, 9, 3);
INSERT INTO Stock VALUES(16, 10, 3);
INSERT INTO Stock VALUES(17, 11, 3);
INSERT INTO Stock VALUES(18, 12, 3);
--find all stock at Import Motors
SELECT * FROM Model
WHERE ModelNumber IN
     (SELECT DISTINCT Stock.ModelNumber
     FROM Stock WHERE Stock.DealerID = (
          SELECT Dealer.DealerID
          FROM Dealer WHERE Dealer.DealerName = 'Import Motors'
--find all dealers with BMW Z3
SELECT * FROM Dealer
WHERE DealerID IN
     (SELECT DISTINCT Stock.DealerID
     FROM Stock WHERE Stock.ModelNumber = (
          SELECT Model.ModelNumber
          FROM Model WHERE Model.ModelName = 'BMW Z3'
     );