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Constraints

Constraints. Primary Key Constraint. It is derived from the entity UID or created as surrogate key. Uniqueness is enforced by an index: Index is not defined in Oracle Designer All columns in key are mandatory. It should not be updatable. Every table should have one!. Unique Key Constraint.

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Constraints

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  1. Constraints

  2. Primary Key Constraint • It is derived from the entity UID or created as surrogate key. • Uniqueness is enforced by an index: • Index is not defined in Oracle Designer • All columns in key are mandatory. • It should not be updatable. • Every table should have one!

  3. Unique Key Constraint • Derived from secondary entity UIDs • Can include NULL columns • Are optional • Recommended where a surrogate key is used for PK

  4. Foreign Key Constraint • Derived from relationships • Can reference either a primary or a unique key: • But avoid referencing unique keys! • Must not be partly NULL • Can reference its own table • Cannot reference tables on other databases

  5. RENTAL TITLE COPY COPY DML DML DML DML RENTAL_ITEM COPY BOOKING BOOKING NULL DML DEFAULT Foreign Key Rules DEFAULT NULLIFY CASCADE RESTRICT (default) • Server handles RESTRICT and CASCADE Delete only X

  6. Defining Foreign Key Rules Foreign Key Properties • Cascades • Defaults • Nullifies • Restricted Cascade Rules Delete Rule Update Rule • Avoid using any update rule: • PKs should not be updatable

  7. EMPLOYEES IDENTIFIER LAST_NAME FIRST_NAME SALARY POSITION Check Constraint CHECK ((position = 'TECHNICAL' AND salary BETWEEN 3000 AND 4000) OR (position != 'TECHNICAL'))

  8. A C ID ID B FK_A FK_B ID CHECK ((FK_A IS NOT NULL AND FK_B IS NULL) OR (FK_B IS NOT NULL AND FK_A IS NULL)) Foreign Keys in Arcs DSD1

  9. Implementing Subtypes GAME and MOVIE subtypes in TITLES TITLES PRODUCT_CODE CHECK ((TI_TYPE = 'GA'AND GAME_CATEGORY IS NOT NULLAND MEDIUM IS NOT NULL AND MOVIE_CATEGORY IS NULL AND AGE_RATING IS NULL) OR ... TI_TYPE TITLE MOVIE_CATEGORY AGE_RATING GAME_CATEGORY MEDIUM

  10. More Uses for Check Constraints • START_DATE < END_DATE • Positive numeric column values > 0 • IS NOT NULL • Can only check columns within the same row; use triggers for more complex checks

  11. Create Check Constraint for EMPLOYEES : Name Check constraint name Check constraint condition Column Defining a Check Constraint • Specify the Where/Validation condition EMP_DEPT_FK

  12. Where Are Constraints Validated? • At the client, the server or both • Considerations: • Database integrity • Interactive response • Network traffic • Overall system performance

  13. Relational Table Definitions Foreign Keys BOO_COP_FK Foreign Key Properties Validation Validate in Where to enforce Client Server Both None Specifying Location of Validation

  14. Action Edit Block Field Customers: DML Posted Server Validation Server Client Guaranteed integrity Minimal network traffic No immediate feedback

  15. Action Edit Block Field Customers: DML Posted Server and Client Validation Server Client Guaranteed integrity Immediate feedback Network traffic for every check

  16. Action Edit Block Field Customers: DMLPosted Client Validation Server Client Immediate feedback No guaranteed integrity Network traffic for every check and again for DML or commit

  17. + + ? Guidelines for Choosing Where to Validate • Server validation: • Guaranteed data integrity • Client and server validation: • Check constraints • Lookup foreign keys • Case-by-case basis: • Other foreign keys • Unique keys

  18. Action Edit Block Field TransactionCommitted Customers: Controlling When to Enforce Client Server • Typical transactions: • Oracle validates each DML statement. • Complex transactions: • Oracle8 server can defer validation until COMMIT.

  19. Relational Table Definitions Foreign Keys BOO_COP_FK Foreign Key Properties Validation Defer Status INITIALLY DEFERRED INITIALLY IMMEDIATE NOT DEFERRED Specifying When Validation Occurs Whentoenforce

  20. Summary • Constraints enforce data integrity: • Entity integrity • Referential integrity • Arcs, subtypes, and simple business rules • Validation can be carried out: • Server and client • Deferred

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