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Database Design

Database Design. Sections 11 Database relationship, Integrity, keys, mapping conceptual model to logical/physical model. Relational database concepts. Discuss Primary keys Foreign keys Data integrity Physical mapping & transition to SQL Entity = table Attribute = column.

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Database Design

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  1. Database Design Sections 11Database relationship, Integrity, keys, mapping conceptual model to logical/physical model

  2. Relational database concepts • Discuss • Primary keys • Foreign keys • Data integrity • Physical mapping & transition to SQL • Entity = table • Attribute = column Marge Hohly

  3. Relational database table Marge Hohly

  4. SQL to retrieve information • Structured query language (SQL) used to access information • English-like phrases • Example: SELECT last_name, department_idFROM employeesWHERE employee_id = 200; Marge Hohly

  5. Results of SQL statement Marge Hohly

  6. Primary Key • Primary Key (PK) • Column or set of columns that uniquely identifies each row in a table • Employee ID in Employee table (single unique) • Bank ID & Account ID in Accounts table (composite) • Every table has a Primary key • not null • no part of PK can be null (entity integrity) • unique • can be composite • Candidate key (column that can be considered for a Primary key) Marge Hohly

  7. Marge Hohly

  8. Foreign Key • Foreign Key (FK) • depends on business rule • comes from relationship • primary key from another table • If FK is part of a PK, then the FK can’t be NULL Marge Hohly

  9. Marge Hohly

  10. Key questions • what makes emp_no and payroll_id good candidates for the primary key? • why is having alternate or unique keys useful? Marge Hohly

  11. Column integrity • Contain values consistent with data format of column Marge Hohly

  12. Summary Data-Integrity Rules Marge Hohly

  13. Data-Integrity Summary Marge Hohly

  14. Data-Integrity Summary • Entity integrity- no part of PK can be NULL • Referential integrity – FK must match an existing PK value (or else be NULL) • Column integrity – column must contain only values consistent with defined data format • User-defined integrity – data stored in database must comply with the rules of the business Marge Hohly

  15. Referential Integrity • Use Foreign Key to map relationships • A foreign key (FK) is a column or combination of columns in one table that refers to a primary key in the same table or another table. • (next slide) Marge Hohly

  16. Example Marge Hohly

  17. Composite key • Made up of two or more values • Together unique • ENROLL Table/Entity • student_no & ticket_no • ACCOUNTS • bank_no & acct_no Marge Hohly

  18. JOBS Table Marge Hohly

  19. Transformation • Conceptual model, focus on the business and its rules. • Data modeling pays attention to the business requirements, regardless of implementation. • Conceptual model Logical model Marge Hohly

  20. Review Marge Hohly

  21. Conceptional becomes Physical model Conceptual becomes Physical model Marge Hohly

  22. - An entity leads to a table. - An attribute becomes a column. - A primary unique identifier produces a primary key. - A secondary unique identifier produces a unique Key. - A relationship is transformed into a foreign key and foreign-key columns. - Constraints are the rules that the database must follow to be consistent. Some of the business rules are translated into check constraints; other more complex ones require additional programming in the database or the application. Terminology Mapping Marge Hohly

  23. For entity names of more than one word, take the: - First character of the first word - First character of the second word - Last character of the last word Example: JOB ASSIGNMENT gets a short name of JAT For entity names of one word but more than one syllable, take the: - First characer of the first syllable - First character of the second syllable - Last character of the last syllable Example: EMPLOYEE gets a short name of EPE For entity names of one syllable but more than one character: - First character - Second character - Last character Example: FLIGHT gets a short name of FLT Short Name Example Marge Hohly

  24. Naming restrictions with Oracle • Table and column names: • must start with a letter • can contain up to 30 alphanumeric characters • cannot contain space or special characters such as “!,” but “$,” “#,” and “_” are permitted • Table names must be unique. • Column names must be unique within a table. • Avoid “reserved” words in tables and columns. Marge Hohly

  25. Cascade barred relationships • UID from parent entity becomes part of the UID of the child entity Marge Hohly

  26. Relationship mapping • Relationships are mapped to foreign keys (at the many end) • Foreign keys enable users to access related information from other tables. • Mapping relationships to relational database structures is part of creating the “first-cut” database design. Marge Hohly

  27. 1:M mapping Foreign key goes in table at crow’s foot from parent FK1 Dept_id mandatory is required FK2 might be better mgn_id and is optional Does the president of the company have a manager? Relationship mapping Marge Hohly

  28. Relationship mapping • FK is mandatory from this diagram • FK is optional from this diagram Marge Hohly

  29. Optional or Mandatory determined by crow’s foot end of relationship Enforcing Optionality Marge Hohly

  30. NonTransferable Relationship • Transferablility is a procedural model • Must be implemented by a program • Need to document this constraint/business rule Marge Hohly

  31. Barred Relationship • Barred relationship is mapped to a foreign-key column on the many side, just like any other M:1 relationship. • Bar means it becomes part of the composite primary key of the child • ACCOUNT table has both acct_id and bank_id as the composite primary key Marge Hohly

  32. Pick up one more component to the composite key with each level Company – company_id Division company_id & div_id Department company_id, div_id & dept_no Team team_id, company_id, div_id & dept_no TEAM within made up of DEPARTMENT within made up of DIVISION within made up of COMPANY Cascading barred relationships Marge Hohly

  33. M:M resolved with intersection entity Intersection entity has a composite key with the PK from each parent as FK in child M:M relationship mapping Marge Hohly

  34. 1:1 relationship mapping • Create a foreign key and a unique key • If relationship mandatory on one side, Foreign key created on the mandatory side as a unique key • If optional on both sides, you can choose which table gets the foreign key. Marge Hohly

  35. Review • FK 1:M • PK, FK in same key, rename one • M:M first resolve with an intersection entity * o Marge Hohly

  36. Review cont. • Will be part of PK a composite key • FK on mandatory side • FK on either side Marge Hohly

  37. Arc mapping • Foreign key from the parent (single) side are placed in the child (many) side • The Foreign key is ALWAYS Optional in the child • Only one of the Arc can be valid and all others must be NULL • Mandatory relationship is enforced with a check constraint Marge Hohly

  38. Marge Hohly

  39. Arc constraint • You need a constraint to make sure only one is NOT NULL at a time • Example: FK1, FK2, FK3, .... • ALTER EVENT constraint (FK1 is not null and FK2 is null and FK3 is null ....) OR (FK1 is null and FK2 is not null and FK3 is null ....) OR (FK1 is null and FK2 is null and FK3 is not null ....) Marge Hohly

  40. ARC mapping • If mandatory then one MUST be NOT NULL • If optional then all may be NOT NULL • You will always need a check constraint defined Marge Hohly

  41. Subtype Review Marge Hohly

  42. Subtype mapping • Mapping supertypes and subtypes makes sure that the right information gets stored with each type. Marge Hohly

  43. Mapping as a single table Rules Tables: Only one table is created, independent of the number of subtypes. Columns: The single table gets a column for all the attributes of the supertype, with the original optionality. Table gets a column for each attribute of the subtype, but column are optional. Mandatory column to distinguish between each different subtypes of entity. Subtype modeling Marge Hohly

  44. Subtype modeling – Single table cont. • Rules • Identifiers: Unique identifiers transform into primary and unique keys. • Relationships: Relationships at the supertype level transform as usual. Relationships at subtype level are implemented as optional foreign-key columns. • Integrity constraints: A check constraint is needed to ensure that for each particular subtype, all columns that come from mandatory attributes are not null. Marge Hohly

  45. Note mandatory attributes salary/hourly rate became optional Need check constraint to enforce mandatory requirement CHECK (epe_type = ‘FTE’ and salary is not null and hourly_rate is null and agy_id is null) OR (epe_type = ‘PTE’ and salary is null and hourly_rate is not null and agy_id is not null) Subtype model – Single table Marge Hohly

  46. When Supertype/Single table • The single-table implementation is common and flexible implementation. • Appropriate where: • Most attributes are at supertype level • Most relationships are at supertype level • Business rules are globally the same for the subtypes Marge Hohly

  47. Create a table for each subtype Rules Tables: One table per first-level subtype. Columns: Each table gets a column for all attributes of the supertype with the original optionality.Each table also gets a column for each attribute belonging to the subtype, also with the original optionality. Identifiers: The primary UID at the supertype level creates a primary key for each table. Secondary UIDs of the supertype become unique keys in each table. Relationships: All tables get a foreign key for a relationship at the supertype level, with the original optionality. For relationships at the subtype levels, the foreign key is implemented in the table it is mapped to. Original optionality is retained. Two-Table implementation Marge Hohly

  48. A separate table would be created for SHIRTS and SHOES. 2-table cont. Marge Hohly

  49. Subtype Considerations • Subtype implementation may be appropriate when: • Subtypes have very little in common. There are few attributes at the supertype level and several at the subtype level. • Most of the relationships are at the subtype level. • Business rules and functionality are quite different between subtypes. • How tables are used is different -- for example, one table is being queried while the other is being updated. Marge Hohly

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