Relational Database Design
Starting from scratch, here’s the general procedure for designing a relational database:
1. Identify the basic “entities” that will be stored in the database. For the actors example from
topic B1, the basic entities would be actors and movies, although in a large database you might
also have tables for studios, directors, and more.
2. Create a table for each entity, making sure that each table has a valid primary key.
3. Verify that each table obeys the golden rule—every non-key field depends on the key, the whole
key, and nothing but the key. If not, it means that you probably need some more tables.
4. Identify the relationships between the tables.
5. For many-to-many relationships, you will have to create correlation tables to link the data.
Database design is as much art as science. Designs which seem complete become incomplete when
more detail is required. For example, if the only information you need about a movie’s director is her
name, you don’t need a Directors’ table. But as soon as you start adding information about the
directors, you’ll need to create a separate directors’ table to avoid breaking the golden rule.
In addition, databases in which data is actively being added (or deleted/modified) are generally designed
differently from historical databases which are not being modified. For historical data, it is common to
move AWAY from third normal form, putting most of the data into one or two tables. This makes
querying the data easier and faster.
Example
Here’s an example of relational database design.
Still-Solvent Motors
This little-known car company quietly churns out a profit year after year, due mainly to phenomenal
database design. Let’s see if we can guess what their dealers’ inventory databases might look like.
The main inventory table will have one row for each car on the dealer’s lot (or in the showroom).
Still-Solvent offers three car models: The SS1, SS2, and SSSUV. Here are their brochure descriptions,
including options available.
•
SS1: A sporty compact car. Seats 4. Available with standard 4-cylinder engine or 6-cylinder fuelinjected engine for more power. Manual or automatic transmission. Options available are air
conditioning, power brakes, power steering, sun roof, driver-side airbag.
•
SS2: A great midsize family car. Seats 5. Available only with 6-cylinder fuel-injected engine and
automatic transmission. Options available are air conditioning, power brakes, power steering,
sun roof, driver-side airbag, side-curtain airbags, rear-seat airbags, IPod/MP3 compatible
surround sound stereo system, GPS Navigation system, anti-lock brakes.
•
SSSUV: Most intimidating SUV on the road. Hummers flee in fear. Seats 14. Available with 6-liter
450 HP 10-cylinder engine or 9-liter 600 HP 16-cylinder engine. 100- gallon gas tank so you only
need to fuel up every 200 miles (financing available). Optional 4WD. Other options available are
air conditioning, power brakes, power steering, sun roof, driver-side airbag, side-curtain airbags,
rear-seat airbags. Choice of IPod/MP3 compatible surround sound stereo system or live band
(including storage compartment), GPS Navigation system, anti-lock brakes, pedestrian catcher,
and top wheels for getting out of those nasty rollovers.
The Tables
•
The base table will be Inventory. The natural primary key for any table of actual cars is the
Vehicle Identification Number, or VIN.
•
The one-to-many fields, as described in the brochure, would include model, engine, and
transmission. For simplicity, we’ll treat engine and transmission as being fully described by a
short text description, so we won’t make tables for them.
•
However, we have important additional information about the models, so we’ll make a separate
Models table.
•
Engine and transmission are one-to-many because while each engine type or transmission might
be in many cars on the lot, each car has only one of each.
•
The other options, however, can be mixed in any combination in any car. A particular option,
like power brakes, will exist in many cars. And a particular car may have many options.
•
Therefore, we will need a correlation table between inventory and options.
Tables Summary
Inventory
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VIN: Primary Key
•
ModelName
•
Engine
•
Transmission
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ModelName: Primary Key
•
Class
Models
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Seats
Options
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OptionID:Primary Key
•
OptionName
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OptionDescription
InventoryOptions
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VIN: Primary Key
•
OptionID: Primary Key
Those tables should be enough to query the inventory to find cars of particular models with particular
options. However, if a customer wants to order a car from the factory, we should include another
correlation table:
ModelOptions

ModelName: Primary Key

OptionID: Primary Key
Final Relationship Diagram: