Peter Brawley and Arthur FullerIt's a 10-step:
There are two problems with this. First, changing address does not change the customer; they're two different sorts of things. Second, many people have more than one address (address during the academic year, summer address, and so on---not to mention affluent customers who have homes in several countries). In database-speak, the relationship from customers to addresses is 1:many. Then a customer table with address columns becomes a customer table and an address table with a customer foreign key. Likewise an orders table with order item columns becomes an orders table and an order items table with an orders foreign key. Simpler, less redundant, more logical.
D: Give each child table a column that references its parent table, to
automate the database logic that keeps these relationships intact. Such a column
is a foreign key. Do not carry IDs up to grandparents; that would
introduce redundancy, thus complicating data management and queries.
E: Configure lookup tables not covered by steps C and D: A child can have multiple parents. Any table in the chain may belong to a table that is not in a main chain. For example counties, schools and departments may each have a computerservicesID columns that points to a computer services table, and a staffID column pointing to a staff table:
CREATE TABLE school (
ID int auto_increment primary key,
name varchar(50) not null Default 'Not known',
universityID int not null references university(ID),
computerservicesID int not null references computerservices(ID),
deanstaffID int not null references staff(ID),
Thus many schools can share a computer service, staff can work in multiple schools, and so on. The diagram becomes more interesting:
university <----- computerservices
| | |
school <---------------+ |
F: Add bridge tables for many-many relationships: Each student
takes multiple tests, and each test is taken by multiple students; that is a many:many
relationship. To implement it, you need a bridge table, a
students_testresults table which has
its own primary key column, plus a foreign key column pointing to a students
row, plus a foreign key column pointing to a particular testresults row.
Steps 4A through 4F bring your database to Third Normal Form, roughly. We can get rid of yet more redundancy, for example by creating a table SchoolNames, and replacing the name column in schools with a schoolnameID that points to the schoolnames row for that name. Now no name is stored twice. Whether to take normalisation that far depends on how important a particular redundancy is for queries, and for available storage space.
In the Requirement (Step 1), underline all the verbs to get a first outline of what actions will have to be performed on the Tables. Then organise those actions into query specifications (eg a shipping clerk begins by looking up an order by date, customer or order number).
6. Build the database from the normalised model
It is no longer necessary to do this by hand—your data modelling tool can often do this with one button click. A terrific open source tool for diagrammiing a database from requirements is the Azzurri Clay modeller for Eclipse; see "Eclipse, an elegant MySQL IDE" here. An alternative is MySQL Workbench.
7. Seed the database with data
Insert test data into the database.
Translate every Action from Step 5 into a query. Notice that this includes all required reports! Test each query.
9. Test, loop and revise as necessary
As you proceed, loop back as far as need be to plug holes and correct errors.
Write no application code till you arrive here.
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