Dec 08, 2014
In this first part (of a 3-part series), we'll look at normalising our database design! The First Normal Form (1NF)
sets the very basic rules for an organised database:
What do these rules mean when contemplating the practical design of a database?
- Eliminate duplicative columns from the same table.
- Create separate tables for each group of related data and identify each row with a unique column (the primary key).
It’s actually quite simple.
The first rule dictates that we must not duplicate data within the same row of a table.
Within the database community, this concept is referred to as the atomicity of a table. Tables that comply with this rule are said to be atomic. Let’s explore this principle with a classic example – a table within a human resources database that stores the manager-subordinate relationship. For the purposes of our example, we’ll impose the business rule that each manager may have one or more subordinates while each subordinate may have only one manager.
Intuitively, when creating a list or spreadsheet to track this information, we might create a table with the following fields:
However, recall the first rule imposed by 1NF: eliminate duplicative columns from the same table. Clearly, the Subordinate1-Subordinate4 columns are duplicative. Take a moment and ponder the problems raised by this scenario. If a manager only has one subordinate – the Subordinate2-Subordinate4 columns are simply wasted storage space (a precious database commodity). Furthermore, imagine the case where a manager already has 4 subordinates – what happens if she takes on another employee? The whole table structure would require modification.
At this point, a second bright idea usually occurs to database novices: We don’t want to have more than one column and we want to allow for a flexible amount of data storage. Let’s try something like this:
- Subordinates, where the Subordinates field contains multiple entries in the form "Mary, Bill, Joe"
This solution is closer, but it also falls short of the mark. The subordinates column is still duplicative and non-atomic. What happens when we need to add or remove a subordinate?
We need to read and write the entire contents of the table. That’s not a big deal in this situation, but what if one manager had one hundred employees? Also, it complicates the process of selecting data from the database in future queries.
Here’s a table that satisfies the first rule of 1NF:
In this case, each subordinate has a single entry, but managers may have multiple entries.
Now, what about the second rule: identify each row with a unique column or set of columns (the primary key)? You might take a look at the table above and suggest the use of the subordinate column as a primary key. In fact, the subordinate column is a good candidate for a primary key due to the fact that our business rules specified that each subordinate may have only one manager. However, the data that we’ve chosen to store in our table makes this a less than ideal solution. What happens if we hire another employee named Jim? How do we store his manager-subordinate relationship in the database?
It’s best to use a truly unique identifier (such as an employee ID) as a primary key.
Our final table would look like this:
- Manager ID
- Subordinate ID
Now, our table is in first normal form! Stay tuned for the second and third parts of this series, where we continue our normalisation!