relational_algebra

relational_algebra

ID:40091422

大小:97.78 KB

页数:31页

时间:2019-07-20

relational_algebra_第1页
relational_algebra_第2页
relational_algebra_第3页
relational_algebra_第4页
relational_algebra_第5页
资源描述:

《relational_algebra》由会员上传分享,免费在线阅读,更多相关内容在学术论文-天天文库

1、QueryingtheDatabase•Goal:specifywhatwewantfromourdatabaseUIUC-employee(SSN,name,address,salary)IRS-IL(SSN,city,year-of-filed-return)•Findalltheemployeeswhoearnmorethan$50,000andpaytaxesinChampaign-Urbana.1QueryingtheDatabase•CouldwriteinC++/Java,butbadidea•Insteadusehigh-le

2、velquerylanguages:–Theoretical:RelationalAlgebra,Datalog–Practical:SQL–Relationalalgebra:abasicsetofoperationsonrelationsthatprovidethebasicprinciples.•Worryfree2Whatisan“Algebra”?•Mathematicalsystemconsistingof:–Operands---variablesorvaluesfromwhichnewvaluescanbeconstructe

3、d.–Operators---symbolsdenotingproceduresthatconstructnewvaluesfromgivenvalues.3Q:Examplealgebra?•Arithmeticalgebra.Linearalgebra.•Whatareoperands?•Whatareoperators?4WhatisRelationalAlgebra?•Analgebrawhoseoperandsarerelationsorvariablesthatrepresentrelations.•Operatorsaredes

4、ignedtodocommonthingsthatweneedtodowithrelationsinadatabase.•Theresultisanalgebrathatcanbeusedasaquerylanguageforrelations.5RelationalAlgebraataGlance•Operators:relationsasinput,newrelationasoutput•FivebasicRAoperations:–BasicSetOperations•union,difference(nointersection,no

5、complement)–Selection:s–Projection:p–CartesianProduct:X•Whenourrelationshaveattributenames:–Renaming:r•Derivedoperations:–Intersection,complement–“Join”s(natural,equi-join,thetajoin)6BasicRAOperations7SetOperations•Union,difference•Binaryoperations8SetOperations:Union•Union

6、:alltuplesinR1orR2•Notation:R1UR2•R1,R2musthavethesameschema•R1UR2hasthesameschemaasR1,R29SetOperations:Difference•Difference:alltuplesinR1andnotinR2•Notation:R1–R2•R1,R2musthavethesameschema•R1-R2hasthesameschemaasR1,R210Selection•Returnsalltupleswhichsatisfyacondition•Not

7、ation:σc(R)•cisacondition(uses=,<,>,and,or,not)•Outputschema:sameasinputschema11SelectionExampleEmployeeSSNNameDepartmentIDSalary999999999John130,000777777777Tony132,000888888888Alice245,000Findallemployeeswithsalarymorethan$40,000.σSalary>40000(Employee)SSNNameDepartmentID

8、Salary888888888Alice245,00012SelectionExampleEmployeeSSNNameDepartmentIDSalary9999

当前文档最多预览五页,下载文档查看全文

此文档下载收益归作者所有

当前文档最多预览五页,下载文档查看全文
温馨提示:
1. 部分包含数学公式或PPT动画的文件,查看预览时可能会显示错乱或异常,文件下载后无此问题,请放心下载。
2. 本文档由用户上传,版权归属用户,天天文库负责整理代发布。如果您对本文档版权有争议请及时联系客服。
3. 下载前请仔细阅读文档内容,确认文档内容符合您的需求后进行下载,若出现内容与标题不符可向本站投诉处理。
4. 下载文档时可能由于网络波动等原因无法下载或下载错误,付费完成后未能成功下载的用户请联系客服处理。
相关文章
更多
相关标签