The One-to-Many Relationship

The one-to-many relationship Entities are related to other entities A 1:m relationship

Hierarchical relationships Occur frequently Multiple 1:m relationships FIRMDIVISIONDEPARTMENTSECTION

Create another entity to avoid update anomalies Insert Delete Update

STOCK with additional columns stock stkcodestkfirmstkpricestkqtystkdivstkpenatnameexchrate FCFreedonia Copper United Kingdom1.00 PTPatagonian Tea United Kingdom1.00 ARAbyssinian Ruby United Kingdom1.00 SLGSri Lankan Gold United Kingdom1.00 ILZIndian Lead &Zinc United Kingdom1.00 BEBurmese Elephant United Kingdom1.00 BSBolivian Sheep United Kingdom1.00 NGNigerian Geese United Kingdom1.00 CSCanadian Sugar United Kingdom1.00 ROFRoyal Ostrich Farms United Kingdom1.00 MGMinnesota Gold USA0.67 GPGeorgia Peach USA0.67 NENarembeen Emu Australia0.46 QDQueensland Diamond Australia0.46 IRIndooroopilly Ruby Australia0.46 BDBombay Duck India

Mapping to a relational database Each entity becomes a table The entity name becomes the table name Each attribute becomes a column Add a column to the table at the many end of a 1:m relationship Put the identifier of the one end in the added column

NATION and STOCK nation natcodenatnameexchrate UKUnited Kingdom1.00 USAUnited States0.67 AUSAustralia0.46 INDIndia stock stkcodestkfirmstkpricestkqtystkdivstkpenatcode FCFreedonia Copper UK PTPatagonian Tea UK ARAbyssinian Ruby UK SLGSri Lankan Gold UK ILZIndian Lead &Zinc UK BEBurmese Elephant UK BSBolivian Sheep UK NGNigerian Geese UK CSCanadian Sugar UK ROFRoyal Ostrich Farms UK MGMinnesota Gold USA GPGeorgia Peach USA NENarembeen Emu AUS QDQueensland Diamond AUS IRIndooroopilly Ruby AUS BDBombay Duck IND

Foreign keys A foreign key is a column that is a primary key of another table –natcode in stock is a foreign key because natcode is the primary key of nation Record a 1:m relationship

Referential integrity constraint For every value of a foreign key there is a primary key with that value For every value of natcode in stock there is a value of natcode in nation A foreign key can never be null A primary key must exist before the foreign key can be defined –Must create the nation before its stocks

Creating the tables CREATE TABLE nation ( natcodeCHAR(3), natnameVARCHAR(20), exchrateDECIMAL(9,5), PRIMARY KEY (natcode)); CREATE TABLE stock ( stkcodeCHAR(3), stkfirmVARCHAR(20), stkpriceDECIMAL(6,2), stkqtyDECIMAL(8), stkdivDECIMAL(5,2), stkpeDECIMAL(5), natcodeCHAR(3), PRIMARY KEY(stkcode), CONSTRAINT fk_stock_nation FOREIGN KEY(natcode) REFERENCES nation ON DELETE RESTRICT);

Representing a 1:m relationship in MS Access

Join Create a new table from two existing tables by matching on a common column SELECT * FROM stock, nation WHERE stock.natcode = nation.natcode; stkcodestkfirmstkpricestkqtystkdivstkpenatcodenatcode1natnameexchrate NENarembeen Emu AUS Australia IRIndooroopilly Ruby AUS Australia QDQueensland Diamond AUS Australia BDBombay Duck IND India ROFRoyal Ostrich Farms UK United Kingdom CSCanadian Sugar UK United Kingdom FCFreedonia Copper UK United Kingdom BSBolivian Sheep UK United Kingdom BEBurmese Elephant UK United Kingdom ILZIndian Lead & Zinc UK United Kingdom SLGSri Lankan Gold UK United Kingdom ARAbyssinian Ruby UK United Kingdom PTPatagonian Tea UK United Kingdom NGNigerian Geese UK United Kingdom MGMinnesota Gold US United States GPGeorgia Peach US United States

Join Report the value of each stock holding in UK pounds. Sort the report by nation and firm. SELECT natname, stkfirm, stkprice, stkqty, exchrate, stkprice*stkqty*exchrate AS stkvalue FROM stock,nation WHERE stock.natcode = nation.natcode ORDER BY natname, stkfirm; natnamestkfirmstkpricestkqtyexchratestkvalue AustraliaIndooroopilly Ruby AustraliaNarembeen Emu AustraliaQueensland Diamond IndiaBombay Duck United KingdomAbyssinian Ruby United KingdomBolivian Sheep United KingdomBurmese Elephant United KingdomCanadian Sugar United KingdomFreedonia Copper United KingdomIndian Lead & Zinc United KingdomNigerian Geese United KingdomPatagonian Tea United KingdomRoyal Ostrich Farms United KingdomSri Lankan Gold United StatesGeorgia Peach United StatesMinnesota Gold

GROUP BY - reporting by groups Report by nation the total value of stockholdings. SELECT natname, SUM(stkprice*stkqty*exchrate) AS stkvalue FROM stock, nation WHERE stock.natcode = nation.natcode GROUP BY natname; natnamestkvalue Australia India United Kingdom United States

HAVING - the WHERE clause of groups Report the total value of stocks for nations with two or more listed stocks. SELECT natname, SUM(stkprice*stkqty*exchrate) AS stkvalue FROM stock, nation WHERE stock.natcode = nation.natcode GROUP BY natname HAVING COUNT(*) >= 2; natnamestkvalue Australia United Kingdom United States

Subqueries A query nested within another query Report the names of all Australian stocks. SELECT stkfirm FROM stock WHERE natcode IN (SELECT natcode FROM nation WHERE natname = 'Australia'); stkfirm Narembeen Emu Queensland Diamond Indooroopilly Ruby

Correlated subquery Solves the inner query many times Find those stocks where the quantity is greater than the average for that country. SELECT natname, stkfirm, stkqty FROM stock, nation WHERE stock.natcode = nation.natcode AND stkqty > (SELECT AVG(stkqty) FROM stock WHERE stock.natcode = nation.natcode) natnamestkfirmstkqty AustraliaQueensland Diamond89251 United KingdomBolivian Sheep United KingdomRoyal Ostrich Farms United StatesMinnesota Gold816122

Views - virtual tables An imaginary table constructed by the DBMS when required Only the definition of the view is stored, not the result CREATE VIEW stkvalue (nation, firm, price, qty, exchrate, value) AS SELECT natname, stkfirm, stkprice, stkqty, exchrate, stkprice*stkqty*exchrate FROM stock, nation WHERE stock.natcode = nation.natcode;

Views - querying Query exactly as if a table SELECT nation, firm, value FROM stkvalue WHERE value > ; nationfirmvalue United KingdomFreedonia Copper United KingdomPatagonian Tea United KingdomAbyssinian Ruby United KingdomSri Lankan Gold United KingdomIndian Lead & Zinc United KingdomBolivian Sheep United KingdomNigerian Geese United KingdomCanadian Sugar United KingdomRoyal Ostrich Farms United StatesMinnesota Gold United StatesGeorgia Peach AustraliaNarembeen Emu AustraliaQueensland Diamond AustraliaIndooroopilly Ruby

Why create a view? Simplify query writing –Calculated columns Restrict access to parts of a table