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Relational data objects

Relational data objects. Lecture 6. Answer to last lecture’s activity. Example used throughout the course. Suppliers s_id (string); s_name (string); status (integer); city (string) Parts p_id (string); p_name (string); colour (string); weight (real); city (string) Contracted

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Relational data objects

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  1. Relational data objects Lecture 6

  2. Answer to last lecture’s activity

  3. Example used throughout the course • Suppliers • s_id (string); s_name (string); status (integer); city (string) • Parts • p_id (string); p_name (string); colour (string); weight (real); city (string) • Contracted • s_id (string); p_id (string); qty (integer)

  4. Diagrammatic representation

  5. Relation Tuples Cardinality Attributes Degree Terminology

  6. Domains

  7. Why domains? • comparisons • price with price OK • weight with weight OK • weight with price !!! • POSSIBLE (they are both numbers) • BUT WRONG, i.e. MEANINGLESS • integrity of the DB

  8. Why domains? - example Suppose the P_id and S_id fields are numeric in all tables SELECT P_name, Qty FROM Parts, Contracted WHERE Parts.P_id = Contracted.P_id SELECT P_name, Qty FROM Parts, Contracted WHERE Parts.P_id = Contracted.S_id meaningless

  9. Domains • domains • are not explicitly stored in the DB • are specified as part of the DB definition (where?) • each attribute - defined on a certain domain

  10. Scalars • the smallest semantic unit of data • atomic values - no internal structure with respect to the relational model • NOTE: they may have internal structure • comparison with a basic data type (Pascal) • non-deterministic definition (“slippery”)

  11. Domain = data + operators • integer: +, -, *, integer-division • real • char: concatenate, find string, … • date: • more complex data types • spatial operators: all regions crossed by a line • images

  12. Domains and data types • domain (in the relational model) is the same with data type in a programming language • analyse the extent to which domain / data type definition can be supported • primitive support for domains in relational databases (INTEGER, CHAR(n), ...)

  13. Data definition • domains • are not explicitly stored in the DB • are specified as part of the DB definition (where?) • each attribute - defined on a certain domain

  14. Conventions • domains • unique in the DB • named relations • unique in the DB • attributes • unique within a relation

  15. Domain based queries • if domains exist • which relations contain information related to… a certain domain (e.g. related to suppliers - S_id) • if domains don’t exist • naming convention + • attribute interrogation

  16. Domains - advantages • domain constrained operators • increased representational power • spatial primitives for GIS; image processing features … • domain based queries

  17. Relations

  18. Variable vs value • variable - named object whose values can change • value - element of a type DECLARE VARIABLE example OF TYPE INTEGER // ... example := 1; // variable has certain value //... example := example * 2; // variable has certain value //...

  19. Relation variable vs relation value • relation variable • named object whose value at a given time is a relation value • its values change with time • relation value CREATE BASE RELATION example //... // insert operations SELECT //... FROM example // the variable has a certain value WHERE //...

  20. Relation value • a relation value on a collection of domains (not necessarily distinct) consists of a heading and a body

  21. Relation heading • a fixed set of attribute_name:domain_name pairs • { <A1:D1>, <A2:D2>, ..., <An:Dn>} • each Ak corresponds to only one Dk • A1, ..., An are all distinct • degree (arity) - n

  22. Relation body • a set of (n-)tuples • n is the same for all the tuples in the set • n-tuple • a set of attribute_name:attribute_value pairs • { <A1:vi1>, <A2:vi2>, ..., <An:vin> } (tuple i) • vik must be from Dk • one and only one pair for each attribute • cardinality - no of tuples

  23. Example - the relation Suppliers heading {<s_id:STRING>, <S_name:STRING>, <Status: INT>, <City:STRING>} body { {<s_id:’s1’>, <s_name:’Smith’>, <status:20>, <city:’London’>}, {<s_id:’s2’>, <s_name:’John’>, <status:30>, <city:’Leeds’>}, {<s_id:’s3’>, <s_name:’Stella’>, <status:70>, <city:’Paris’>} }

  24. Generate some incorrect examples

  25. Tables as relations • relation table • a table can be interpreted as a relation • there are some underlying domains • each column corresponds to a certain domain • the table’s heading represents the relation’s heading • each row represents a tuple, etc.

  26. Properties of relations • no duplicate tuples • SQL!!! • consequence - primary key • tuples are un-ordered • attributes are un-ordered • all attributes are atomic • 1NF; why do we need 1NF? • unnormalised relation - example

  27. Kinds of relations named relation base relation derived relation expressible relation query result intermediate result view (virtual) snapshot (real) stored relation

  28. Relational database • “a database that is perceived by the user as a collection of normalised relations of assorted degrees” (Codd, 1983)

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