© Copyright 2002, L. M. Linson, may be freely used with this notice Practical Database Design “Structuring Your Tables” by Larry Linson, presented to the Microsoft Access SIG of the North Texas PC User Group January 12, 2002

© Copyright 2002, L. M. Linson, may be freely used with this notice Purpose Laying out or structuring tables –in an Access (or other relational) database Without formal data modeling Without becoming a relational database expert Accomplishing results with –common sense –intuition

© Copyright 2002, L. M. Linson, may be freely used with this notice “Relational” “Relation” = Table – Rows (“Tuples”)and – Columns (“Attribute”) Scalar means one value per … Not from “Relationships” – between Tables

© Copyright 2002, L. M. Linson, may be freely used with this notice Heading of a Relation Label for each attribute Name : Domain Name identifies Domain = kind of data (not the data type)

© Copyright 2002, L. M. Linson, may be freely used with this notice First Normal Form If domains are scalar – one item of information per attribute – some questionable, e.g., dates clearly consist of parts depends on problem space do you need to use the parts?

© Copyright 2002, L. M. Linson, may be freely used with this notice Second Normal Form In First Normal Form and If all attributes dependent on the primary key – no “extraneous” information – Invoice Invoice Number Supplier Name Supplier Phone Item Price

© Copyright 2002, L. M. Linson, may be freely used with this notice Third Normal Form... In Second Normal Form and If all attributes are mutually independent – violation example, If one Sales Rep per State – Order Order ID Sales Rep State Customer ID

© Copyright 2002, L. M. Linson, may be freely used with this notice Third Normal Form In practical terms, as far as we usually go, and sometimes useful to violate continued from previous slide

© Copyright 2002, L. M. Linson, may be freely used with this notice There must be only one authoritative source for any item of information.

© Copyright 2002, L. M. Linson, may be freely used with this notice

and it will force you into a structured, usable file design

© Copyright 2002, L. M. Linson, may be freely used with this notice Avoid: Redundancy … A BAD IDEA – Wastes resources – Makes life harder because – You have to store multiple copies – Which is correct?

© Copyright 2002, L. M. Linson, may be freely used with this notice Avoid: Redundancy … Even worse if – in two or more tables because – not only how many records to change – but how many places to look for it

© Copyright 2002, L. M. Linson, may be freely used with this notice Avoid: Composite Fields Concatenating is easy – [Title] &[First] & [Mid] & [Last] & [Suf] Extracting isn’t so easy – “Umm? How?” he pondered, scratching his head and staring

© Copyright 2002, L. M. Linson, may be freely used with this notice Avoid: Repeated Fields One record w/ repeat – [Student] – [Period 1 Class] – [Period 2 Class] –... – [Period 6 Class] Which students study biology?

© Copyright 2002, L. M. Linson, may be freely used with this notice Our Example... A friend, client, or employer says, –“Our order-taking process is a little clumsy. I’d like you to create a database to smooth it out and make it quicker for the sales people.”

© Copyright 2002, L. M. Linson, may be freely used with this notice Our Example... You ask, –“What, exactly, do you want in that database?” continued from previous slide

© Copyright 2002, L. M. Linson, may be freely used with this notice Our Example Friend, client, employer responds, –“Here’s one of our order forms. Too, I’d like to keep track of the product training courses our sales people have to take. Please make a prototype to suggest what I need.” continued from previous slide

© Copyright 2002, L. M. Linson, may be freely used with this notice An Order Customer Name and Number Bill-To Address Ship-To Address Quantity, Product, Price, Extended Price... Order Number, Date Ship Method & Date Salesperson Subtotal, Tax, Shipping, Total

© Copyright 2002, L. M. Linson, may be freely used with this notice Better than we might expect Suggests the information Seems to group the information, too Natural and intuitive –Sales staff used to it –Likely refined over years Try using it to group the information –Then make tables for each group

© Copyright 2002, L. M. Linson, may be freely used with this notice Not Just “Order Information” Customer Name and Number Bill-To Address Ship-To Address Quantity, Product, Price, Extended Price... Order Number, Date Ship Method & Date Salesperson Subtotal, Tax, Shipping, Total

© Copyright 2002, L. M. Linson, may be freely used with this notice Some “Customer” Info Customer Name and Number Bill-To Address Ship-To Address Quantity, Product, Price, Extended Price... Order Number, Date Ship Method & Date Salesperson Subtotal, Tax, Shipping, Total

© Copyright 2002, L. M. Linson, may be freely used with this notice Some “Salesperson” Info Customer Name and Number Bill-To Address Ship-To Address Quantity, Product, Price, Extended Price... Order Number, Date Ship Method & Date Salesperson Subtotal, Tax, Shipping, Total

© Copyright 2002, L. M. Linson, may be freely used with this notice Repeated “Detail” Info Customer Name and Number Bill-To Address Ship-To Address Quantity, Product, Price, Extended Price... Order Number, Date Ship Method & Date Salesperson Subtotal, Tax, Shipping, Total

© Copyright 2002, L. M. Linson, may be freely used with this notice Some Product Info Customer Name and Number Bill-To Address Ship-To Address Quantity... Order Number, Date Ship Method & Date Salesperson Subtotal, Tax, Shipping, Total Extended PriceProduct, Price

© Copyright 2002, L. M. Linson, may be freely used with this notice And the rest is “Order Info” Customer Name and Number Bill-To Address Ship-To Address Quantity... Order Number, Date Ship Method & Date Salesperson Subtotal, Tax, Shipping, Total Extended PriceProduct, Price

© Copyright 2002, L. M. Linson, may be freely used with this notice Grouping by Intuition Salespeople Customers Orders Order Details Products

© Copyright 2002, L. M. Linson, may be freely used with this notice Adding the Training Salespeople Customers Orders Order Details Products Training

© Copyright 2002, L. M. Linson, may be freely used with this notice Sales Database Information – The Intuitive Approach Salespeople sell products to customers Customers purchase products Orders record a customer purchase Order Details show products, quantities Products are what is sold, with price Training (Product information) for Salespeople

© Copyright 2002, L. M. Linson, may be freely used with this notice Try Laying Out Tables Customers Products Order Details Orders Salespeople Training

© Copyright 2002, L. M. Linson, may be freely used with this notice Rearrange by Relationship Customers Products Orders Salespeople Training (use real-world relationships) There’s a customer for every order There’s a salesperson for every order Order Details

© Copyright 2002, L. M. Linson, may be freely used with this notice Continue Rearranging Customers Products Orders Salespeople Training A customer can have many orders A salesperson can have many orders Order Details

© Copyright 2002, L. M. Linson, may be freely used with this notice Continue Rearranging Customers Products Order Details Orders Salespeople Training An order can have many order details

© Copyright 2002, L. M. Linson, may be freely used with this notice Continue Rearranging Customers Products Order Details Orders Salespeople Training A product can be in many order details

© Copyright 2002, L. M. Linson, may be freely used with this notice Continue Rearranging Customers Products Order Details Orders Salespeople Training A salesperson can take many training classes

© Copyright 2002, L. M. Linson, may be freely used with this notice Did We Forget Something? Customers Products Order Details Orders Salespeople Training Many salespeople can be in a training class Maybe! Depends on what we may need to do.

© Copyright 2002, L. M. Linson, may be freely used with this notice Did We Forget Something? Customers Products Order Details Orders Salespeople Training Many salespeople can have taken a training class or many can be in a session of a class. No, let’s revise that statement:

© Copyright 2002, L. M. Linson, may be freely used with this notice Did We Forget Something? Customers Products Order Details Orders Salespeople Training Many salespeople can have taken a training class or many can be in a session of a class. If the first, then we have a many-to-many

© Copyright 2002, L. M. Linson, may be freely used with this notice Did We Forget Something? Customers Products Order Details Orders Salespeople Training If we have a many-to-many, to implement we need a junction or intersection table Junction

© Copyright 2002, L. M. Linson, may be freely used with this notice Did We Forget Something? Customers Products Order Details Orders Salespeople Training Many salespeople can have taken a training class or many can be in a session of a class. If our data model requires the second, then we need another table for sessions.

© Copyright 2002, L. M. Linson, may be freely used with this notice How Many Levels Take to as many levels of detail –as you need to represent what you need to “model” A database is merely a model –of the real world –detailed enough to satisfy the needs and deal with the business “issue” –we don’t have “problems” any more, only “issues” –or, perhaps, “opportunities”

© Copyright 2002, L. M. Linson, may be freely used with this notice How Many Levels? Einstein is supposed to have said, “As simple as possible, but no simpler.” Too simple, and you don’t satisfy the purpose Not simple enough, it will be “cumber- some” and you have to work too hard to satisfy the purpose

© Copyright 2002, L. M. Linson, may be freely used with this notice What did he say Look at what is needed You are “modeling” the real world Base on “real world” activity, data Organize for what you need Look for natural groupings Look for natural relationships Review with the user

© Copyright 2002, L. M. Linson, may be freely used with this notice Thank you for your kind attention Please, gently wake your neighbor before leaving Come again when you can

© Copyright 2002, L. M. Linson, may be freely used with this notice Practical Database Design “Structuring Your Tables” The End