CS609

Introduction

Databases

• Current state?

• Future?

Relational Model  (1970  E.F. Codd) - Chapters 2-3• A data model:•

        types         relationships         constraints         basic operations

• relational model - files + mathematical foundation (set theory)

        Relation - Table         Rows - Tuples         Attributes - Columns

Relational Rules

1.  1NF

2.  Access rows by content only (can't say 3rd row), unique rows

3.  Integrity constraints • key constraints (all tuples distinct) • entity constraints - no primary key null • domain constraint - defines domain values • referential constraint - if a tuple refers to

another tuple, it must be an existing tuples • semantic constraint - defines rules for

relationships

Notation

• Relational algebra (algebraic notation) and relational calculus - logical notation– used to demonstrate potential for query

language of relational DB – algebra and calculus equivalent in expressive

power

Operations

• Set theoretic ops:– union, intersection, difference, cartesian product

• Native relational ops:– project, select, division, join - equi, theta, natural join– R / S - result is those rows t such that for every row s

in S, if concatenate t and s, can be found in R

• Other ops

– outer joins - left, right, aggregate (set) functions, grouping

Operations cont’d

• Precedence of relational algebra ops:  (in order of highest) – Project– Select– cartesian product– join & divide– Minus– Union– intersection

Complete Set

• Can express any query (except aggregate functions) with the complete set – Select – Project – Cartesian product X – Union U – Difference -

    R/S = R - ((R X S) - R) given R(A1...An,B1...Bm) S(B1...Bm)

(A1...AnR) - A1...An((A1...AnR X S) - R)R ^ S = (RUS)-((R-S) U (S-R))

Query Language• SQL or SEQUEL - (Structured English Query Language) • High-level DB language created at IBM  in 1970's

        DB system products using SQL released in early 80's • Provides DML and DDL• Standard SQL

– SQL-86, SQL-89 - minimal standard, still incompatibility product features– ANSI/ISO - (SQL-92) SQL2 - tried to avoid incompatibility problems– X/Open standard - extensions plus portability– SQL-3– Current standard SQL:1999, SQL-99– Future standard SQL:2003, SQL:200n     

                          

SQL• Subselect - formal definition:         Select expr {, expr}

        From tablename [alias] {, tablename [alias]}         [Where search_condition]         [Group By col {, col}]         [Having search_condition]

• Full Select - formal definition:      Subselect {Union [all] subselect}

  [Order By result_col [asc | desc] {, result_col [asc | desc]}]

http://download-west.oracle.com/docs/cd/A91202_01/901_doc/server.901/a90125/statements_103.htm#2080289

Sample SQL queries

• List the ssn for each female employee and pname of projects she works_on.

• List the ssn and salary for all employees who do not have a son as a dependent.

• List lname, fname of employees who have more than 2 dependents.

• List lname, pname, plocation for all projects worked on by femail employees with > 2 dependents.

• List employee ssn and salary (with 10% increase) for all employees who do not work on project # 30.

• Give every employee a 10% raise.

Power/Weakness of SQL

•  SQL is a non-procedural language – user specifies what is desired rather than how

(e.g.  no need to write loop statement to traverse tuples)

– no implied ordering – query optimizer decides how – desired result described in one statement – Avoids procedural complexities – A language with the power of relational algebra

defined as relationally complete (Codd)

Weakness of SQL

• 1) Too many equivalent forms – e.g.  in, =any, exists

            not in, <> all

• 2) No non-procedural language can have Turing power (computational complete), e.g. perform any computational procedure that

can be specified in algorithmic terms – Not true of SQL only relationally complete

Weakness

• 3) Some capabilities missing– Selects cannot be arbitrarily nested like

relational algebra – loss of expressive power of a relational

algebra expression

• 4) Cannot nest set functions – e.g. Average of total dollar sales

• 5) No statistical functions (besides avg)– e.g.  variance, mode

Weakness• 6) Can find reports SQL can't create - need embedded

SQL – e.g.  categorize total sales based on size of sale

• 7) No Transitive closure - arbitrary number – managers of managers

• 8) Limited by Boolean conditions – Cannot retrieve 5 out of 6 – Cannot list 20 agents with largest sales – No distance to keywords

• Are 1-8 still true of ORACLE?

Strengths

• While selects cannot be arbitrarily nested - have added power in search (where) condition

• Other strengths of SQL?