Description and exemplification use of a Data Dictionary. A data dictionary is a catalogue of all data items in a system. The data dictionary stores details and descriptions of all of data items. The data dictionary is usually developed after normalisation has been carried out and helps the analyst in determining system requirements. A data dictionary is used to fully describe all data items that are held in a system.

Description and exemplification use of a Data Dictionary. The ERD does not give any indication of the type or size of each attribute nor does it show the restrictions that apply to an attribute or where else in the system the attribute is being used. These details are held in a properly developed data dictionary. The details of each data time that must be included in a data dictionary are: – –attribute name – –entity – –type – –size – –validation – –index/key

Description and exemplification use of a Data Dictionary. Item Name – –It is good practice to make sure that all item names in a data dictionary are unique. One way to do this is to incorporate the name of the entity within the item name. This ensures that any attributes that have more than one entry in the data dictionary (for example, foreign keys) are easily distinguishable. Data Type – –Common data types indicated in a data dictionary include: number (used to store any value that consists of numbers only) text (used to store any value that is made up of characters or characters combined with number) date (used to store any value that represents a calendar date) time (used to store any value that represents a time of the day) auto (used to indicate that the value is a numeric that the system generates automatically)

Description and exemplification use of a Data Dictionary. Data Size – –The size of a data item refers to the maximum number of characters that will be allowable. This only needs to be considered when the data type is text. – –It is important to consider the size of each data item since the default set by most development tools are often far larger than the sizes necessary. By setting an appropriate maximum size, the analyst is reducing the amount of wasted memory that the system will use when implemented. – –When a data item requires no more than characters, it is extremely inefficient to allow the database software to set a default size of, say 50.

Description and exemplification use of a Data Dictionary. Foreign Keys – –Note that the data type and size of any foreign keys must match the type and size set for the original primary value. In particular, if a foreign key has a primary value that has been set as an auto value, then the foreign key type must be set as number.

Description and exemplification use of a Data Dictionary. Validation – –The validation columns of the data dictionary should indicate any restrictions that should be applied to the data item when values are being entered into the system. – –Validation checks automatically check any input values to make sure that they are sensible. Common validation checks that are indicated in a data dictionary include: presence check – can a null value be used range check - values entered are within certain pre-defined upper and lower limits restricted choice -only certain values can be entered lookup - existing (primary) value of any foreign key data item

Description and exemplification use of a Data Dictionary. Index and Key – –The Index/Key column of the data dictionary is used to indicate whether or not the data item is to be indexed and if it is to be indexed, whether or not the data item is a primary key or a foreign key. – –Indexing Database systems take advantage of indexing to increase their speed. A database index can speed up a query by hundreds or thousands of times. Indexing is the notion of storing data on a hard disk in a particular way in order to locate and retrieve the data as efficiently as possible. – –Keys When a data item is marked as indexed, the analyst must indicate if indexing is due to the fact that the data item is a primary key (PK) or if it is because the data item is a foreign key (FK). Where a foreign key is part of a primary key, both of these facts should be indicated in the data dictionary.

Description and exemplification of entity/event modelling. Index and Key – –The Index/Key column of the data dictionary is used to indicate whether or not the data item is to be indexed and if it is to be indexed, whether or not the data item is a primary key or a foreign key. – –Indexing Database systems take advantage of indexing to increase their speed. A database index can speed up a query by hundreds or thousands of times. Indexing is the notion of storing data on a hard disk in a particular way in order to locate and retrieve the data as efficiently as possible. – –Keys When a data item is marked as indexed, the analyst must indicate if indexing is due to the fact that the data item is a primary key (PK) or if it is because the data item is a foreign key (FK). Where a foreign key is part of a primary key, both of these facts should be indicated in the data dictionary.

Description and exemplification of entity/event modelling. There are two types of entity/event modelling: –Entity Event Matrix & –Entity Life History.

Description and exemplification of entity/event modelling. Purpose of an Entity Event Matrix – –An Entity Event Matrix is used to record a list of all permissible events that can occur and shows the effect that these events have on the entities within the system. – –In this context, an event is something that triggers a process into updating data within the system. An effect is the change caused by the event such as the creation, deletion or modification of an entity occurrence.

Description and exemplification of entity/event modelling. One single Entity Event Matrix is created for the entire system. This matrix lists all entities in the system across the top of the matrix. The events that occur are listed on separate rows of the matrix. Within the matrix, the effect caused by an event is recorded as follows: – –C This is used to indicate that the described event causes a new entity occurrence (or new row of the table) to be created within the entity indicated. – –D This is used to indicate that the described event causes an existing entity occurrence (or existing row of the table) to be deleted from the entity indicated. – –M This is used to indicate that the described event causes existing data values within the entity indicated to be modified. – –R This is used to indicate that the described event causes data values held within the entity indicated to be read by the process that is triggered by the event. Note that read activity is only possible because of the use of foreign keys to link one entity to another. When a data value in an entity is updated, it is possible any entity that has a foreign key link to that entity to see the update - in other words, the linked entity can read the updating that takes place in the main entity. Read activity is also needed to carry out calculations and to produce invoices, statements and reports.