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CS2208 II Databases - Coggle Diagram
CS2208 II Databases
Intro. To DBMS
Purpose + key features
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Storage, retrieval and updates
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Who uses a database?
Application developers - They may need to use the database to manage transactions and log-ins for users
DBMS managers - They are responsible for the planning, development and implementation of the database
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UML representation
Strong + Weak elements - strong elements are entities that can exist without any others(i.e on their own), weak entities are entities that have to have another entity to exist
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Normalisation
Anomolies
Update anomalies
if we update the address of one branch we must update every instance if that branch held within the database unless it becomes inconsistent and an update anomaly arises
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Deletion anomalies
if you were to have a branch and branch location in the database with the employee data also held in it when a employee leaves a branch therby making the branch completely empty it the deletes all the branch data anong with it
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Insertion anomalies
In a realistic scenario a database for employee data, if the primary key is the name and you have an open position to fill thereby having to make the name "null", there will be an anomaly as the primary key will be null
This anomaly can occur when inserting into a table data that wont be compatible with the design of the table
Normalisation is a technique that is used to produce a good set of relations with desirable properties. They should ensure maintainability take up the minimum amount of storage and not cause anomalies
ER representation
Example
“FutureLeases has many branches around the world.
Every branch has staff. Some staff manage a branch;
this includes supervising staff.”
"Branch" and "Staff" go in as elements represented as a diagram and the relationship between them are arrows linking them
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Every element also has properties/attributes like a branch has "opening hours", "address", "name" etc. and every member of staff has an "ID", "department", "salary" etc.
one of these properties has to be a "primary key" - a primary key is an attribute that is unique to that specific row in the table like an "ID" or "Location" key