Defining Data Flows Data Dictionary

1 DFDs use simple labels describing content of data store or data flow –do not include detail data descriptions Detail descriptions are required for all data –on input forms and documents –on output screens and reports –in data files or database Details of data are held in a Data Dictionary

Data Dictionary –central store of data that supports other models –unambiguous and concise way of recording data about data (metadata) –can be recorded manually or using a software tool –encourages consistency between models by using same names in different models –prevents duplication of data –resolves problems of aliases which are all recorded against the appropriate data item –built up as the models are developed –aids communication as everyone on development team knows the exact meaning of words and terms used

2 Structure of Data Data is structured as follows: –Label in DFDs are data groups –data group consists of one or more data items (data items = data elements or attributes) –data group can also be grouped into other data groups

CustomerDetails = - data group Mr A. T. Green- Name 14 Derry Road- Address Dun Laoghaire Co Dublin Phone No. Name = - data group Mr - Title A.T. - Initial (>1?) Green - Surname Address = - data group 14 Derry Road- Address Line (>1?) Dun Laoghaire Co Dublin Break groups down to constituent data items

3 Notation There are several different notations for recording data dictionaries All must have a way of expressing –sequence in which data items occur –data items which repeat –optional data items –where one of a certain group or items is selected –a possible set of values for a data item –comments

Sequence, selection, iteration Sequence : + –CustomerDetails = Name + Address + PhoneNo Repetition : { } –Name = Title + {Initial} + Surname Optionality : ( ) –CustomerDetails = Name + Address +(PhoneNo) Selection : […|…] – Name = (Title) + [ {ForeName} | {Initial} ] + Surname Values : “…” –Title = [ “Dr” | “Mr” | “Mrs” | “Ms” ] Comments : *….*

4 Example Full data dictionary entries for CustomerDetails Address = {AddressLine} CustomerDetails = Name + Address + (PhoneNo) Name = (Title) + [ {ForeName} | {Initial} ] + Surname Title = [ “Dr” | “Mr” | “Mrs” | “Ms” ]

Data Dictionary conventions –for data flows written in lower case with first letter in capitals where there is more than one word, each word starts with a capital and spaces are removed –for data stores written in capitals where there is more than one word, words are joined with underscore ‘_’ character –stored in alphabetical order

5 Describing Data Items A data item is the smallest unit of data in the dictionary, –e.g. ForeName, Initial, PhoneNo Data items need to be described fully –textual description of the item –information about its format –range of permissible values –aliases –indication of where the data item is used

Format of a data item = Picture –type - numeric, character... –size - number of characters, numbers … –special format characters Examples –Forename -X(20) –Initial-X(1) –PhoneNo-9(12) –Date -99/99/ XXX-9999

Data Flows in the Level 1 DFD Customer Choice Payment Price Docket Financial Report Supply needs Supply order Supply invoice Supply receipts Supply payment

Joe’s Docket

Data Flow Definitions CustomerChoice = {StockCode + QuantityRequired} Docket (stage 1) = DocketNo + {StockCode + QuantityRequired +(StockDescription) + UnitPrice + 20%VAT + TotalPrice} + TotalAmtDue + DocketPaidSig Payment = PaymentAmount Price = StockCode + UnitPrice Docket (stage 2) = DocketNo + {StockCode + QuantityRequired +(StockDescription) + UnitPrice + 20%VAT + TotalPrice} + TotalAmtDue + DocketPaidSig +StaffNo + StaffName + StaffRole + ItemsRecvdSig Financial Report = ??? Not yet defined

More Data Flow Definitions Supply needs = {StockDescription + StockCode + QuantityRequired} We need to change the name of this item, because it is the same name as the quantity required by the customer, but it means something different. We’ll call it StockRequired. Supply needs = {StockDescription + StockCode + StockRequired} Supply order = SupplierInfo + OrderDate + {StockCode + (StockDescription) + StockRequired}

More Data Flow Definitions SupplyInvoice = SupplierInfo + OrderDate + DeliveredDate + {StockCode + (StockDescription) + QuantitySupplied + UnitCostPrice} + VAT + TotalPrice SupplyReceipts = {OrderDate +StockCode + (StockDescription) + QuantitySupplied + UnitCostPrice} SupplyPayment = SupplierInfo + SupplyPayment