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...An R) -  A1...An ((  A1...An R 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]}] west.oracle.com/docs/cd/A91202_01/901_doc/server.901/a90125/statemen ts_103.htm#

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 f 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?