Catalla & cea


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Catalla & cea

  1. 1. INTRODUCTION There are some instances when a primary key doesn’t exist in the real world or when the existingnatural key might not be a suitable primary key.
  2. 2. SURROGATE KEY:Is a primary key created bythe database designer tosimplify the identification ofentity instances.
  3. 3. ADVANTAGESince it has no intrinsicmeaning, values for it can begenerated by the DBMS to ensurethat unique values are alwaysprovided.
  4. 4. TABLE 5.4 DATE TIME_START TIME_END ROOM EVENT_NAME PARTY_OF6/17/2010 11:00AM 2:00PM Allure Burton 60 Wedding6/17/2010 11:00AM 2:00PM Bonanza Adams Office 126/17/2010 3:00PM 5:30PM Allure Smith Family 156/17/2010 3:30PM 5:30PM Bonanza Adam’s Office 126/18/2010 1:00PM 3:00PM Bonanza Boy Scouts 336/18/2010 11:00AM 2:00PM Allure March of 25 Dines6/18/2010 11:00AM 12:30PM Bonanza Smith Family 12
  5. 5.  Given the data shown in table 5.4 you would model the EVENT entity as:EVENT(DATE, TIME_START, TIME_END,ROOM ,EVENT_NAME,PARTY_OF)
  6. 6. DATE TIME_ TIME ROOM EVENT_ PART START _EN NAME Y_OF What primary key would you D suggest? In this case , there is no6/17 11:00 2:00 Allure Burton 60 simple natural key that could be /10 AM PM Wedding used as a primary key in the6/17/ 11:00 2:00 Bonanza Adams 12 model. Based on the primary key 10 AM PM Office concepts you learned about in previous chapters, you might6/17/ 3:00 5:30 Allure Smith 15 suggest one of these options. 10 PM PM Family6/17/ 3:30 5:30 Bonanza Adam’s 12 10 PM PM Office (DATE, TIME_START, ROOM)6/18/ 1:00 3:00 Bonanza Boy 33 Or 10 PM PM Scouts (DATE, TIME_END, ROOM)6/18/ 11:00 2:00 Allure March of 25 10 AM PM Dines6/18/ 11:00 12:3 Bonanza Smith 12 10 AM 0PM Family
  7. 7. DAT TIME_ TIME ROOM EVENT_ PART E START _EN NAME Y_OF Assume you select the D composite primary key (DATE, TIME_START,ROOM) for the6/17 11:00 2:00P Allure Burton 60 EVENT entity. Next ,you/10 AM M Wedding determine that one EVENT may6/17 11:00 2:00P Bonan Adams 12 use many RESOURCES such as/10 AM M za Office tables, projectors, pc and stands, and that the same6/17 3:00 5:30P Allure Smith 15 RESOURCE may be use for/10 PM M Family many EVENTS. The RESOURCE entity would be represented by6/17 3:30 5:30P Bonan Adam’s 12 the ff. attributes:/10 PM M za Office6/18 1:00 3:00P Bonan Boy 33 RESOURCE/10 PM M za Scouts (RSC_ID, RSC_DESCRIPTIO6/18 11:00 2:00P Allure March of 25 N,RSC_TYPE, RSC_QTY,RS/10 AM M Dines C_PRICE)6/18 11:00 12:30 Bonan Smith 12/10 AM PM za Family
  8. 8. DATE TIME_ TIME ROOM EVENT PART START _EN _NAME Y_OF Given the business rules, the M:N D relationship between RESOURCE and EVENT would be represented6/17 11:00 2:00 Allure Burton 60 via the EVNTRSC composite entity/10 AM PM Weddi with a composite primary key as ng follows:6/17 11:00 2:00 Bonanza Adams 12/10 AM PM Office EVNTRSC (DATE, TIME _START, ROOM, RSC_ID, QTY6/17 3:00 5:30 Allure Smith 15 _USED/10 PM PM Family You now have a lengthy four- attribute composite primary key.6/17 3:30 5:30 Bonanza Adam’s 12 What would happen if the/10 PM PM Office EVNTRSC entity’s primary key were inherited by another existence dependent entity? At6/18 1:00 3:00 Bonanza Boy 33 this point, you can see that the/10 PM PM Scouts composite primary key could6/18 11:00 2:00 Allure March 25 make the implementation of the/10 AM PM of database and program coding Dines unnecessarily complex.6/18 11:00 12:3 Bonanza Smith 12
  9. 9. Surrogate primary keys are accepted practice in todays complex data environments.
  10. 10. They are especially helpful when there is no natural key, when the selected candidate key has embedded semantic contents or when the selected candidate key is too long or cumbersome.
  11. 11. However , there is a trade-off; if you use a surrogate key, you must ensure that the candidate key of the entity in question performs properly through the use of “unique index” and “not null” constraints.