Databases

Databases

1. Keywords

   • Persistent simply means that it is stored on secondary storage so that it can be accessed later.
   • Related it has some kind of similarity with someother data
   • Organised there some kind of struture system to the way something works
   • Fields a characteristic of something in a database.
   • External information end user will see
   • Conceptual the links beween tables / dev pov
   • Physical raw data
   • Serial file items are stored as a series one after another.
   • Sequential files the data is in some sequence
   • Flat file only one table aka it only go by surnames
   • Hierarchical a system of teirs and levels

2. Types of databases Databases can be computerised or paper based. An address book is an example of a paper based database. In an address book entries are stored in a logical order, normally by surname and each entry contains the same fields such as name, house number, street name etc.

3. • A Primary Key contains a unique identifier for each record. To make each record in a database unique we normally assign them a primary key. Even if a record is deleted from a database, the primary key will not be used again. The primary key can be automatically generated and will normally just be a unique number or mix of numbers and letters.

4. Validation checks that the correct type of data is entered Verification checks that the data is actually the data you want.

5. A relational database divides the data within the database into different tables with each table focusing on a particular entity. This ensures that most data is stored only once. For example, a newsagents stores information on its customers. It also needs to store information on the newspapers and magazines each customer receives. As this information is a different entity we need to have a separate table for this data.

   • Primary Key Each table should have a primary key. This is a field chosen to be unique for each record. Usually this is some sort of reference number. Foreign key A foreign key is used to link tables together and create a relationship between them. It is a primary key in one table that is linked to a field in another table.

6. Most databases are manipulated with a Database Management System (DBMS). A DBMS provides the software tools to create, maintain and interrogate a database as well as providing an interface to applications.

   • What tasks does a DBMS have to carry out? • allow different applications to access data at the same time; • Controls access to the data. Security features are provided to limit who can do what. • allow multiple user access at the same time; • Controls concurrency- meaning that data is locked while someone is working on it so that someone else cannot change the same data at the same time. • hold just one copy of data that everyone can access – this prevents inconsistent updates; • provides user interfaces to work with the data in the required view • makes it possible to change the data in a database without having to change the applications. • creates backups and the ability to restore from a backup in the event of a disaster • Support a query language to enable extracting, adding and amending its data. • Force referential integrity. This means that it prevents operations that could damage relationships. Such as deleting a subject from a school database when students are linked to t
   • features
     • Queries Queries are objects that extract data from a database. They are used to select the fields required and the conditions that need to be met. A query can be created by using an interactive graphic tool called “Query by example” QBE.
     • Forms This is a user friendly ways of allowing the user to see and modify. Without these sql would take programming expertise.
     • Reports These are often the printed out of data from the database, they can be tables or fully formed and well-presented reports. They are usual based on a queries.

7. SQL

   • One of the strengths of a DBMS is the query language called SQL, that it supports. The screen shot below shows a table from a database. The table is called employees.
   • SELECT * FROM Employees
   • A query however normally wants to select only records in a table that meet a set condition. We can achieve this in SQL in much the same way as in QBE. SELECT * FROM Employees WHERE { some logical condition}

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