Chapter 6—Data Flow Diagramming
DFD symbols
1. Processes
2. External Entities
Labeled with verb phrases – Prepare sales order or Mail
Invoice.
Labeled with noun phrases – Vendor , Customer
3. Data Flows
Labeled with noun phrases – remittance advice, customer order
4. Data stores
Labeled with noun phrases – Accounts payable master file.
DFD design conventions
1. All processes should have unique names
2. The inputs to a process should differ from the outputs to a process
3. Any single DFD should not have more than about seven processes
4. No process can have only outputs
5. No process can have only inputs
6. A process has a verb phrase label
7. Data must be moved by a process from one data store to another data store
8. Data cannot move directly from an external entity to a data store
9. Data must be moved by a process to an external entity from a data store
10. A data store has a noun phrase label
11. Data of any concern to the system cannot move directly between external entities
12. An external entity has a noun phrase label
13. A data flow has only one direction between symbols
14. A data flow cannot go directly back to the same process it leaves
15. A data flow can go directly into a data store
16. A data flow has a noun phrase label
Comparison with flowcharts
1. Symbols
a. DFDs have four: circles, rectangles, lines and parallel lines
b. Flowcharts have many, including rectangles, diamonds and triangles
2. Organization
a. DFDs are in leveled sets, each depicting more detail than the previous level
b. Flowcharts are organized in columns which depict areas of responsibility, such as
departments
3. Numbers
a. In DFDs, processes are numbered based on their level. Process 1.0 can lead to
Processes 1.1 and 1.2; Process 1.1 can lead to Processes 1.1.1 and 1.1.2
b. In flowcharts, numbers are used for on- and off-page connectors
4. Focus
a. DFDs focus on data and how it moves between elements of an information
system
b. Flowcharts are concerned with data, but also with documents and processing
tools
5. Use of lines
a. In a data flow diagram, lines represent data. They are labeled with noun phrases
b. In a flowchart, lines represent movement between processes, areas of
responsibility and the like. They are not labeled
Database design
1. Fundamental design element: the table
a. Rows are “records”
b. Columns are “fields”
c. A record usually comprises many fields
d. A field captures a single information item, like a date, last name or invoice
number
2. Keys
a. Each record is uniquely identified by its primary key.
i. Student ID number
ii. Textbook ISBN
b. If a single field alone won’t suffice, a table can have a compound primary key.
i. Last name and first name
ii. Department, course code and section
c. A foreign key is a primary key that appears in another table.
3. Other design elements
a. Queries: sets of instructions for retrieving data, performing calculations and
other tasks
i. Display all sales invoice numbers from January 1 to January 15
ii. Calculate the total amount due from customers whose last names start
with “C”
4. Other design elements
a. Forms: used for entering and looking up data
i. Order forms on the Internet
ii. Looking up class schedules on your school’s information system
b. Reports: used to display data
5. Do not store derivable data in a table.
6. Data normalization
a. The set of rules used to ensure that databases are as efficient and effective as
possible
b. Minimize database size, optimize table design and access data more quickly and
efficiently
c. Six normal forms exist; the first three are usually enough for business
applications
i. 1st normal form (1NF): eliminate repeating groups
ii. 2nd normal form (2NF): eliminate repeating groups AND eliminate
redundant data
iii. 3rd normal form (3NF); eliminate repeating groups, redundant data AND
columns not dependent on primary key