How is information stored?

Transcription

1 How is information stored? Stored in system files on disk Sequential Files Random Access Files Need to write programs to access, modify, append data

2 Example: Bank account system Program to debit/credit an account Program to add new a account Program to find balance of an account Program to generate monthly statements Can use a file system, with application programs written to handle above tasks

3 Problem with such a system What if the bank wanted to introduce a new type of account (which allows overdrafts, say) What if the bank required a list of all accounts with a certain characteristic? (can write another application program, or do it manually)

4 What are the other problems? Data redundancy and inconsistency Difficulty in accessing data Integrity problems Atomicity problems Concurrent-access problems Security problems

5 So what is a data base? Data stored as a set of files - Methods to access data - Methods to insert data - Methods to delete data - Methods to modify data What s the difference?

6 How data is stored? Data is stored in files on disk A set if indices is constructed to facilitate access How these indices are stored (in a data structure) determines the efficiency of data access

7 Introduction to Data Bases A Data Base is a collection of tables Each table is a grid (number of rows and columns) The tables are often interrelated (overlapping columns) Such a collection is called a relational data base

8 City Country City Pop (1995) City Pop (2015) Country pop ( 95) Currency Beijing China Yuan Shanghai China Yuan Tianjin China Yuan

9 Problem with approach Too much replication of data If many cities are from same country, then all the country data replicated How do you get around this? Have two tables, one containing information for cities, and one containing information for countries

10 Example Suppose we wanted to store (and access) the following information related to all the major cities in the world Name of city Population of city (1995) Projected population of city (2015) Country in which it is situated Population of country (1995) Currency used in the country

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13 Pros and cons of two tables No replication of data (efficient in terms of data storage an issue for large data bases) More difficult to extract certain information e.g. if we had to find out the list of cities and the currency used, we have to consult two tables The data base `engine does the job for us

14 Relational data base A collection of usually related tables that has been created using database management software The database software stores these tables so that access is efficient (using fancy data structures, such as B-Trees) We will not worry about these issues here

15 Visual Basic interface to DBs Through data control Data controls can read records Modify records Add records Delete records

16 Data control Data controls do not display data

17 Displaying data Need to `hook the data control to the data base Can use other visual basic controls to display data Need to use `data-aware controls Need to bind the control to the data control Need to tell what table (or part of table) to get information from

18 A data control walkthrough The DatabaseName property should be set to the name of the data base file The RecordSource property should be set to the name of the table that is to be displayed The actual display occurs only when a dataaware control is installed on the form

19 RecordSet Type 0 Table 1 Dynaset 2 Snapshot The first one means that one of the tables that exists in the data base is returned The other two mean that a `view (as specified by an SQL query) is returned

20 RecordSource property Need not be the name of a table that exists in the data base Could be a `view of the table, or combination of tables A view is a subset of a table A view is a subset of a new table (temporarily created) obtained by combining the data from more than one table Such a view is specified by an SQL `query

21 Example of setting up a data control

22 Displaying data Place a text box on the form Set the DataSource property of the text box to the name of the data control Set the DataField property of the text box to the name of the field (column) of the table you want the text box to display Can have more than one text box tied to the data control

23 Modifying the data base Change the value in the text box displaying the data Go to another record by clicking the arrow key in the data control Only works if the `locked property is set to `false Set locked property to true to prevent modification of the data base

24 Other properties of data control Exclusive only one user can access data base at any time Connect - specifies the type of data base RecordSetType Table, Dynaset, or Snapshot Set to Snapshot if you do not want the user to modify the data base These properties can also be set at run time

25 Introduction to SQL English-like statements to select records from one or more tables in a relational data base according to a criteria specified in the SQL statement A query consists of the SQL keyword SELECT followed by one of the keywords WHERE, SELECT, FROM, HAVING, GROUP, ORDER BY

26 SQL Statements Can also occur in code thus: Data1.RecordSource = SELECT FROM e.g. SELECT city FROM Cities Can also use SQL statement to select a set of columns from combination of tables

27 SQL Statement SELECT * FROM Cities, Countries WHERE Cities.Country = Countries.Country Select all fields (columns) from a `view which combines the Cities table and the Countries table SELECT City, Currency FROM Cities, Countries WHERE Cities.Country = Countries.Country

28 Code to select a current record In the data control, we use the navigation arrows to select a new current record Code can also be used to select a new current record Data1.RecordSet.MoveFirst Data1.RecordSet.MoveLast Data1.RecordSet.MovePrevious Data1.RecordSet.MoveNext