mysql - SQL Whatisadatabase? 4 acollectionofdata 4

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SQL
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What is a database? a  collection of data Usually  consists of  entities  and  relations An  entity  is an individual “object” that exists and is distinguishable  from other individuals. Example:  specific person, company, event, plant Entities have  attributes Example: people have  names  and  addresses A relationship is an association among several entities
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Database Management  System (DBMS) A computerized record-keeping system Allows operations such as: Adding new files Inserting data into existing files Retrieving data from existing files Changing data Deleting data Removing existing files from the  database
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Data Models data model  is a collection of concepts for  describing data. schema  is a description of a  particular  collection of data, using the given data model. The  relational model  of data  is the  most  widely used model today. Main concept:  relation , basically a table with rows  and columns. Every relation has a  schema , which  describes the  columns, or fields.
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Levels of Abstraction
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Relational Databases Data is logically perceived as a two- dimensional table Relational databases are sets of tables tables are relations
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Example Table
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Relational Database Query A relational database  query  is a question  about the data, and the  answer consists of a  new relation containing the result. Queries are one  part of the  Data  Manipulation  Language  of a DBMS (we  also  need to create, update, insert data) Language:  Structured Query Language  (SQL)
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Example SQL query Select G.Accession, G.Medline From Genebank G Where G.source=`baker’s yeast’;
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No explicit links between  tables Of  course, there may be  implicit  links in  that two  tables share the same attribute  (like the accession  number) In fact,  in a relational DB, this is the  only  way to  connect distinct tables, at  the logical level anyway   A link  between one table and another is  called a  foreign key
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This note was uploaded on 03/02/2012 for the course C S 360 taught by Professor Clement,m during the Winter '08 term at BYU.

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mysql - SQL Whatisadatabase? 4 acollectionofdata 4

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