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Organisation Of Data (1) Database Theory
11.3 Organisation of Data for Effective Retrieval Describe the nature and purpose of a relational database and how it works. Understand that data needs to be organised in a relational database to allow for effective updating and retrieval. Understand how data can be retrieved to produce meaningful information. Recall the relevant advantages of relational databases over flat file information storage and retrieval systems. Select and justify appropriate file and relational database structures for particular applications.
What Is A Database? An organised collection of data / information. Not necessarily stored on a computer. A computerised database gives: –flexibility in organising, displaying, and printing –much faster than a manual system.
Data Field: data held on a person can be split into a number of fields; these could be: –ID; first name; second name; address; town; post code; date-of-birth; etc. Record: a collection of all the fields above on one person. File: a collection of records; a payroll file will contain a record for each person on the payroll. Database: may contain many different files, structured in such a way that information can be obtained from one or all of them at once.
Flat File And Relational A computerised database is one of two different types: –Flat-File which is only suitable for a few simple applications. –Relational which is much more comprehensive and flexible and suitable for most applications.
Advantages Of Computerised Databases Only have to enter the data once, all other applications can use this data. Files are linked so an update in one will affect the data in another. The structure can be changed. Access is very rapid. Complex searches can be saved and used over and over again or amended. Data remains consistent as the same data is accessed by everyone. Data can be validated upon entry protecting integrity.
Disadvantages Dependant on the technology, a break in service means no applications can be run. Centrally held data needs extra security than dispersed data. Users need to be trained which is costly.
Flat File Similar to a computerised card box of files. Only one file of records can be stored. Ok for i.e. a list of names and addresses such as what you would find in an address book. No good for businesses where data is more complex and flexibility is required.
Employee Number: 1Employee First name: FredEmployee Surname: Bassett Sex:MAge:21Position:Sales Assistant Salary:£15,000Cafe Address Bridge StreetNew- Cross London com
Relational Database These types are better than the flat-file as everything is not stored in one file, it is stored in several files with relationships between them. The relationship ensures the data contained in one file can be combined and used with data from any or all of the other files. Files in this type of database are known as tables, when designing we create tables containing the various Fields of data.
Advantages Data is not duplicated, less data stored. Time is saved entering data. Processing and searching time reduced.
Consider the flat file database of employee details. Much of the data appears in every record, the cafe details. Instead of repeating these in each record, a file would be created for the cafe details. One field will need to stay as the common link between files, i.e. the employee number. With more data than shown it may be possible to create more than two files.
Employees Employee Title Employee First Name Employee Surname Employee Number Address Road Address Town Post Code Employee Telephone Employee Salary Employee Sex Employee Age Employee Date of Birth Employee Position Cafe Name Cafe Address Road Cafe Address Town Cafe Address Post Code Cafe Telephone Cafe Employee Title First Name Surname Number Road Town Post Code Telephone Salary Sex Age DoB Position Cafe Cafe Name Road Town Post Code Telephone Employee Number = +
Instead of entering the cafe details every time data is entered for an employee, each cafe need only to be entered once. The cafe details are kept in the Cafe table and the Employee details are kept in the Employees table. One linking field needs to be present, in this example the employee number. Generally more data would be held in a database and consequently more tables may be required.
Searching The concept of storing data in a database is to retrieve bits of it when required. This is done by searching for data that satisfies a particular criteria, i.e. who are the employees who live in Greenwich. Searches in a database may be done by programming, called Structured Query Language (SQL), or in Access can be done by selecting criteria, called Query By Example (QBE). Access converts QBE into SQL.
Structured Query Language Query By Example (QBE) and Structured Query Language are both industry standard methods for extracting information from a database. The advantage of QBE is that it is graphical and simple to use, the advantage of SQL is that it is almost universally used in a relational database.
Query By Example
A Query A query may: combine the information from one or many related tables, select the fields to be shown, specify which criteria for each field to search for, save the query for repeated use, save the results of a query, or enter them on a report for printing off.
Using SQL SELECT TblEmployee.Title, TblEmployee.[First Name], TblEmployee.Surname, TblEmployee.Position FROM TblEmployee WHERE (((TblEmployee.Position)="manager")) ORDER BY TblEmployee.Surname;
Database Structure The building blocks are the tables and the relationships between the tables. A table is a collection of data arranged in rows and columns. Fields are the titles to the columns. Records are the rows, which contain the set of data for a single item.
Primary Key A field where the data entered is not duplicated in any other record for that field, (unique data). It is usually a numeric field, or a mix of text and numbers, i.e. National Insurance number. In some cases a surname could be used as a Primary key, depending upon the amount of data held, but a second person with the same surname cannot be entered. This could be overcome by having two fields entered as Primary key: First name and Surname. If no Primary key is entered then the database may create one by default: (an Auto-number).
Foreign Key This is a Field in a table, and this Field also exists in another table as the Primary key. This is the basis of a direct link between the two tables. It is the only Field where the data needs to be repeated. What would a suitable Primary key be for: –an employee in the payroll, –a hospital out-patient, –a catalogue of DVD films?
Field Data Types Data entered into a database may be of several types. This helps with the validation of data entered into the database and restricts the entering of erroneous data. Calculations can be performed on numeric data only. The length of text fields need to be regulated to free up allocated space and need to be similar for linking through relationships.
Indexing Important for sorting and searching. Data can be shown in i.e. alphabetical surname order, employee number order, or a combination of two or more Fields if indexed in the design. If there is a Primary key present the database will be sorted by that Field automatically. An index field is generally known as a Secondary key. Surname Indexed Only text, numeric, currency or date can be indexed.
Validation Is done to ensure (as far as possible) that every data entry is accurate. There are several types of validation check: Does it have to be there; Is it within a specified range; Is it one of a list of valid values; Is it in a specified format of numbers and letters?