Class 5 - Cardinality: whatd the biggest number We show...

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Class 5 There is data redundancy Backing up the files is a problem because they don’t know how many files there are Of what does the data fail to be independent? o The data fails to be independent from the programs Every program that is going to create or read that data needs to be given that description When you make a single change to data you have to make 15 other changes on 15 programs If you store data in more than one place, it is only a matter of time before the data’s wrong in at least one of them. Data redundancy will be virtually eliminated When you only store it once you eliminate data inconsistencies When you change the data you change the stored data. When you change the stored data all others change This is more complicated The data is only available through database management software (DBMS) If you forget to change it, you forget to change all of it
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Unformatted text preview: Cardinality: whatd the biggest number We show primary keys: UNDERLINED We show foreign keys: [BRACKETS] o Example: Invoices(Invoice # , Date, [Customer #] Hierarchical block handler: first database available o It could only deal with 1:1 and 1:M relationship (parent-child) o Single root entry point. You could only enter at one point o Explicit Pointer- helps to navigate you around a database Network: another database Ted Cod: try to design a better database o Based on relations (simple chunks of data) Consists of ruples(rows) and attributes (columns) Relationship between tables is done implicitly. We look at the data to see which ones match up For a long time relational databases were just a good idea Relation names must be unique Every relation must have a primary key Duplicate rows and columns are not allowed...
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Class 5 - Cardinality: whatd the biggest number We show...

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