Flashcards
9.3 Data modelling tools
Flashcards do conjunto (36)
- 1What are Data Flow Diagrams (DFD)?They show the flow, storage and processing of data in a system.
- 2Why are DFD's important?They are used to consider the data, while ignoring the equipment used to store it. They are used as a first step in describing a system.
A series of symbols are used in these diagrams but, people use different symbols and this can be confusing to understand if inexperienced. - 3What shape is used to show a Process or Action?A process or action is represented using a rectangular box and it shows a process that does something with the data.
- 4What shape is used to show the External source of data or an External sink of data?This is represented by an oval box which is used to describe where, outside the system, the data comes from and goes to.
- 5What shape is used to show the Data Flow?This is represented by an arrow pointing in the direction of the flow. Usually a description of the flow is written above the direction of the flow so that it can be more efficiently understood.
- 6What shape is used to show the Storage of Data?This can be anywhere data is stored, it can be a drawer, where you keep letters or a filing cabinet. The shape usually has a letter placed in front of the number to show what the data stored is.
E.g. M for manual store and C for a computer store. - 7Why must DFDs be drawn at different levels?When analysing systems, it is usual to draw DFDs at different levels as it reflects the depth in with the DFD looks at the system being investigated.
- 8What is the Context Diagram?A diagram which represents a system as a single DFD process.
- 9What does the Context Diagram do?It is a high-level DFD which shows the entire system under investigation as a single box and the information flows between external entities sch as suppliers and customers.
The aim of the context diagram is to show the scope of the system. - 10What is a Level 1 DFD?This shows the main processes in the system and presents a more detailed view of the data flows in the system. This should not have more than 6 process boxes within the diagram.
- 11What is a Level 2 DFD?A level 2 DFD breaks down the process boxes in the level 1 DFD and thus presents an even more detailed view of the processes. This is beneficial because we can use various diagrams to see the information flow throughout the system in detail, rather than having one big diagram.
- 12When can DFDs be used?During system investigation to record findings.
During system design to illustrate how a proposed system will work.
When outlining the specifications of new systems. - 13What is an Entity?An object of the real world that is relevant to an ICT system. E.g. A place, object, person, customer, invoice, product, course, etc.
- 14What is an Entity Relationship Model (ERMs)?It is a technique for defining the information needs of an organisation to provide a firm foundation on which an appropriate system is built.
An Entity Model is an abstract representation of the data in an organisation and their aim is to produce an accurate model of the information needs of an organisation. This will aid either the development of a new system or the enhancement of an existing one.
An ERM describes a system as a set of data entities with relationships between them (One-to-one, one-to-many, many-to-many). - 15What is an Attribute?A single item of data which represents a fact about an entity. An attribute can be thought of as something which adds further detail/information to the entity.
E.g. The entity STUDENT could have the following attributes: Student Number, Student Name, Date of Birth, Tutor, Address, etc. - 16What are Relationships?A relationship is the way in which entities in a system are related to one another. A relationship can be: One-to-one (1:1), One-to-many (1:m), or Many-to-many (M:n).
- 17How can you decide on the type of relationship?This is best done by looking at the entity from both sides. For example, look at the relationship between students and courses in a college. So, from the student side, a student can take many courses and from the courses side, a course can be taken by many students. So, the relationship between course and student is many-to-many.
- 18What are Keys?Keys are attributes that have special significance. The two main types of keys are: Primary keys and Foreign keys.
- 19What is a Primary Key?It is usually one or several attributes that can uniquely define a particular entity.
For example, the customer entity could be uniquely defined by a unique attribute such as customer number. The payroll entity for example, could be uniquely defined by the entity employee number. - 20What is a Foreign Key?When one of the attributes that is a primary key in one entity also appears in another entity, it means that there is a relationship between the entities.
In the other entity the attribute is not a primary key because it may no longer be unique. So, it is known as a foreign key. - 21What are Atomic Attributes?Atomic attributes are attributes that do not need to be broken down any further.
For example, a beginner new to systems analysis and database design might use an attribute called Name. However, this needs to be broken down into attributes such as first name and surname. - 22Why is Creating the database structure important?Once the problem of many-to-many relationships have been solved, you can think about the tables used to store the data.
Each table can be given the same name as the entities and the field names can be the same as the names given to the attributes. You then have to plan out he structure of each table separately. - 23What is an important note to consider with relationships?In a many-to-many relationship decomposed to a one-to-many and a many-to-one using a link table, the link table should have a Compound Key (E.g. Customer_ID).
Always make sure that the keys used uniquely define a row in the table. - 24What is a Data Dictionary?A centralised collection of information about data such as meaning, relationships to other data, origin, usage, etc., used mainly in database design to ensure consistency.
- 25When are Data Dictionaries used?A data dictionary system is a tool used during system analysis and during database design. The purpose of a data dictionary is to provide information about the database, its uses and participants in the system. A data dictionary could be said to provide "Data about data".
- 26What are the main features included in a Data Dictionary?Entity names
Relationships between the entities
Attribute names
Synonyms
Data type
Format
Description of the attribute
Attribute length
The names of other entities in which the attribute appears - 27Why are Entity Names included in a Data Dictionary?Each entity needs to be given a name and these names need to be recorded and described in the data dictionary.
- 28Why are Relationships between the entities included in a Data Dictionary?The relationships between the entities can be described and then shown in an Entity Relationship Diagram (ERD).
- 29Why are Attribute Names included in a Data Dictionary?Each attribute should be given a name, and these names should be chosen so that they describe the data as fully as possible without being too long. It is best to avoid leaving spaces in field names, using - or _ to separate the words instead.
- 30Why are Synonyms included in a Data Dictionary?Synonyms are alternative names for the same thing. Many large organisations use databases regularly and it is common to find users in different departments employing a different name for the same concept.
However, this can be confusing for systems analysts trying to design and build a new system. To prevent confusion, users should list any alternative names in the synonyms section of the data dictionary. - 31Why are Data Types included in a Data Dictionary?A data type needs to be specified for each attribute. it is very important that once an attribute's data type is set, it has the same data type wherever it occurs. In most cases, relationships can be made only between attributes with exactly the same attribute name and data types.
- 32Why are Formats included in a Data Dictionary?Details of formats should be included in the data dictionary for those attributes which have formats set by the user. For example, a numeric field can be a short integer, long integer, currency, etc., and dates can be given as 12 June 2001, 12/06/01, etc.
- 33Why is the Description of the attribute included in a Data Dictionary?Each attribute should have a description, and these descriptions can be transferred to this section of the data dictionary.
- 34Why is the Attribute Length included in a Data Dictionary?This specifies the length of the attribute for those attributes that can have their lengths set.
- 35Why are the Names of the other entities in which the attribute appears included in a Data Dictionary?An attribute may appear in more than one entity. For example, the attribute Order_Number could appear in the entities ORDER and ORDER_LINE. If you wanted to change the name of an attribute then it is useful to know the entities where it is repeated.
- 36What are some Optional features of the Data Dictionary?Validation checks
Details of the validation checks performed on data entered for each attribute
A key
The type of key specified
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- A level ICT - A Level FlashCards sobre 9.3 Data modelling tools, criado por Cameron Taylor em 30-05-2018.
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- Cameron Taylor
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