Thursday, February 26, 2009

The Normal Forms: First Normal Form (1NF)

First Normal Form (1NF)

In Normal Forms: Introduction, I introduced the topic of the Normal Forms, the theory behind the arrangement of fields into tables. Since it is usually best to start at the beginning, I'll begin with the First Normal Form.

The First Normal Form, or 1NF, is the very lowest, basic arrangement of fields in a table. If your table is not in 1NF, then it isn't really a table. Sadly, many novice databases are not even in 1NF.

A table is said to be in First Normal Form if:
1) there is no row or column order
2) each row (record) is unique
3) each row by column value (field) contains exactly one value
4) there are no repeating columns

What does this mean?

First of all, the operation of the table will be unaffected by the order the rows are in or the order the fields are within the row. It means that each record must be complete unto itself without referencing another row positionally, for example, the row above. Likewise the position of the fields is irrelevant.

Since each record is unique, it means there are no duplicate records. This uniqueness is defined by a field or combination of fields whose value will never be duplicated. This is called the primary key. In order to assure uniqueness, no part of a primary key may be NULL.

Because a field must have a single value, it cannot contain a list or compound value. One over looked consequence of this rule is that each field MUST have at least one value. If the value of the field is not known, it is said to be NULL. (There is some debate over whether NULL is actually a value. I maintain it is, but the discussion is largely semantic.)

Lastly, there are no repeating columns. Repeating columns are columns that store essentially the same information. They may be columns like Product1, Product2, Product3; or multiple Yes/No columns that represent the same information like each product having its own column (Saw, Hammer, Nails).


Let's take a look at how these rules are implemented and what they mean for table design.

Suppose I want a simple Order table with OrderNum, CustomerNum, OrderDate, Quantity, Item, and ProductNum. Although the definition of 1NF is fairly simple, it precludes a wide range of data arrangements. Let's take a look at some of these arrangements.

Figure 1 shows one way such data can be arranged.

Figure 1: Records with Missing Values

To make each record unique, the primary key would have to be OrderNum and Item. However, since no part of the primary key may be Null, this arrangement won't work. All the values of the primary key must be filled in.

But even more than this, the record is not "complete" unto itself. That is, it refers to other records for information. It's not that the values of OrderNum, CustomerNum, or OrderDate are unknown and therefore NULL. I do know the value, but I'm attempting to represent that data positionally. This, of course, violates the first rule (order is irrelevant) and rule 3 (each field must have a value).

This arrangement is common in spreadsheets and reports, but it is not sufficient for storing data.
Figure 2 shows another way the data can be arranged.

Figure 2: Information Stored In Lists

This violates rule 3. Each field must hold one and only one piece of information and not a list. It would be a nightmare to do anything with the date in the Item field other than simply display it because the database management system is designed to treat fields as indivisible.

While Figure 2 is an extreme example that mixes multiple fields in addition to multiple field values, believe it or not, I have also seen database designed like Figure 3:

Figure 3: Data stored in multiple lists

While this is better than Figure 2 (at least it does not mix fields), it is still not atomic and you'd have difficultly associating a quantity with a particular product.

Compound Values:
1NF also precludes compound values, things like full names in a single field or multi-part identification numbers.

Full Names

Why not store a full name? Roger J. Carlson is certainly pertinent information about me. However, it is not indivisible. It is made up of a first name, middle initial, and last name. Because I may want to access just pieces of it (using the first name in the salutation of a letter or sorting by last name), the information should be stored in separate fields.

Multi-part Numbers

Often, a database requirement is to have an identification number that is composed of different, meaningful parts. A serial number may have a four-digit product code, followed by the manufacture date (8 digits), and ended with the facility ID. It might look like this COMP02222008BMH. While this may be a useful arrangement for humans, it is useless in a database. Each value should be stored in a separate field. When the serial number is needed, it can be concatenated easily enough in a query, form, or report.

Figure 4 shows data that is stored in repeated columns.

Figure 4: Data Stored in Repeated Columns

This arrangement is common for people who use spreadsheets a lot. In fact, this is so common it is called "committing spreadsheet". The problem, in addition to having multiple columns, is that in order to associate a quantity with a product, you would have to do it positionally, breaking rule 1.

Lastly, another version of the Repeated Columns error is multiple Yes/No columns. Figure 5 illustrates that.

Figure 5: Data Stored in Yes/No Columns

At first blush, this does not seem to have the same problem, but all I've done is replace generic field names (Product1, Product2, etc) with specific ones (wench, saw, etc). It would be extremely easy to check a second field in any row and they you would have no idea which was correct.


As we've seen, First Normal Form precludes a lot of possible data arrangements. So what's left? There's really only one possibility left. Figure 6 shows it.

Figure 6: 1NF Correct with Repeated Rows

Each row has a unique identifier and there are no duplicates. Each field contains a single value. The position of the row and field is irrelevant, and lastly there are no repeating columns.

It's perfect. Right? Well, no. While this table does conform to 1NF, it is still has some problems; problems that 1NF is not equipped to handle. For those, I need to look at the Second Normal Form (2NF), which is what I'll do next time.


Wednesday, February 18, 2009

The Normal Forms: Introduction

Normalization is a methodology for minimizing redundancy in a database without losing information. It is the theory behind the arrangement of attributes into relations. The rules which govern these arrangements are called Normal Forms.

In What Is Normalization: Parts I, II, III, IV and V, I discussed the decomposition method of normalization, where you put all your fields into a single table and break the them down into smaller, normalized tables.

In Entity-Relationship Diagramming: Parts I, II, III, and IV, I discussed an alternate method which works from the bottom up. It takes the individual pieces of information (Attributes) and group them into logical groupings (Entities).

However, in neither case did I formally define or explain the Normal Forms. And that's for good reason. I find that only after people get a working understanding of normalization do they really understand the Normal Forms and what they imply. Therefore I usually leave them until last. If you haven't read the above mentioned serie, it would be worth your while to do so.

Normalization was first developed by E. F. Codd, the father of Relational Database theory. He created a series of "Normal Forms", which mathematically defined the rules for normalization. Each normal form is "stronger" than the previous, that is, they build upon the previous normal forms. Second Normal Form (2NF) encompasses all the rules of First Normal Form (1NF) plus adding its own rules. Third Normal Form encompasses all of 1NF and 2NF, and so on.

Figure 1: Each Normal Form Encompasses the Previous

In order, the normal forms are: First Normal Form (1NF), Second Normal Form (2NF), Third Normal Form (3NF), Boyce-Codd Normal Form (BCNF), Fourth Normal Form (BCNF), Fifth Normal Form (5NF), and Domain Key Normal Form (DKNF). BCNF comes between 3NF and 4NF because it was developed later, but because of its "strength" belonged between 3NF and 4NF.

Since each normal form encompasses all previous forms, in theory, the higher the normal form, the "better" the database.

In practice, however, normalizing to the first three normals form will avoid the vast majority of database design problems. So it is generally agreed that to be properly normalized, most databases must be in 3NF.

Beyond 3NF, the normal forms become increasingly specialized. Boyce-Codd Normal form and Fourth Normal Form were created to handle special situations. Fifth Normal Form and Domain-Key Normal Form are largely of theoretical intererst and little used in practical design.

So what I'm going to do for this series is limit myself to the first three normal forms, giving their definitions, implications for data, and how to implement them.

In my next post, I'll start with the First Normal Form.

Monday, February 9, 2009

Entity-Relationship Diagramming: Part IV

In Entity-Relationship Parts I, II, and III, I described a method of normalization that builds up from individual data elements. Following this method results in a database design that is normalized to the Third Normal Form -- without having to know what the Normal Forms are.

(I haven't discussed the Normal Forms. Perhaps I will in a later post.)

In Part III, I left off with the completed Entity-Relationship Diagram shown in Figure 1.

Figure 1: Employee Database E-R Diagram

When I do E-R diagramming, I like to ignore the attributes, concentrating instead on just the entities and their relationships. But at some point, obviously, the attributes have to come back into it. I like to save that to the end.

There are some good E-R diagramming tools available out there, like Visio and ERWin, but for small projects, I usually just implement the tables directly in Access and create the Relationships there. The Access Relationship Window works pretty well as a diagramming tool, and you've got a completed database at the end.

Implementing an E-R Diagram

So far, I've talked about Entities and Attributes. I do that deliberately to keep myself from thinking about implementation issues while I'm designing the data model. But at the implementation phase, entities become tables and attributes become fields.

The first thing I do is add an Autonumber, Primary Key field (Surrogate Key), to each table, naming it after the table with a suffix of "ID". Thus the tblEmployee table will have a field EmployeeID. Then I create a Unique Index on the Natural Key I identified during E-R diagramming process.

At this point, my tables look like Figure 2.

Figure 2: Tables with Surrogate Keys Added

Now it's time to look at my relationships. Relationships are created on fields holding related information, Primary Key to Foreign Key. In a One-to-Many (1:M) relationship, the primary key of the table on the "One" side is added to the table on the "Many" side table and becomes the foreign key.

Since there is a 1:M relationship between tblEmployee and tblEmployeeCertification, I'll put EmployeeID in EmployeeCertification and it will become the foreign key for tblEmployee.

I'll do the same with every relationship. The resultant tables look like Figure 3.

Figure 3: Tables with Foreign Keys Added

Lastly, I need to create the actual relationships. I do this by clicking and dragging the primary key of one table into its corresponding foreign key in the other. For instance, I'll click on EmployeeID in tblEmployee and drag it to EmployeeID in tblJobHistory.

When I do, I will get an "Edit Relationships" dialog box shown in Figure 4.

Figure 4: Edit Relationships Dialog Box

The Referential Integrity box will not be checked by default. However, you MUST check it in order to create a true relationship. Without it, Access will create a line between the tables, but it will NOT be a relationship.

When you click the Referential Integrity box, Access should correctly identify the relationship as a One-to-Many relationship. If it does not, if it says Indeterminate, you've done something wrong.

Clicking OK results in the relationship being created. Figure 5 shows that.

Figure 5: Relationship Between tblEmployee and tblJobHistory.

Now, I'll just do that for the rest of the relationships, and I'm done.

Figure 6: Completed Data Model

The process I've outlined here is really just the bare bones. If you're interested in seeing examples in more detail, you can find some in the Tutorials: Database Design section of my website.

Link to top: Entity-Relationship Diagramming: Part IV


Tuesday, February 3, 2009

Entity-Relationship Diagramming: Part III

In this Entity-Relationship series, I'm attempting to present an alternative to standard, decomposition-style normalization.

In Entity-Relationship Diagramming: Part I, I grouped all the objects in my business narrative into either Entities or Attributes. Entities are groups of Attributes and Attribute describe Entities.

In Entity-Relationship Diagramming Part II, I defined the relationships between my entities. Relationships store information about how your Entities interact. Figure 1 shows where I left off.

Figure 1: Relationships Between Entities.

Unfortunately, I'm not done yet, for two reasons: 1) as I said in What is Normalization: Part V, many-to-many relationships cannot be directly implemented in a relational database, and 2) I still have an unassigned attribute. So first I'll rationalize the many-to-many relationship and then take another look.

To rationalize a many-to-many relationship between two tables, you create a third table -- an "intersection" or "linking" table. Then you create one-to-many relationships between the linking table and each of the main tables, with the "many-side" of both relationships on the linking table.

As you can see above, Employee and Certifications have a many-to-many relationship, so I need to create a new entity (Employee/Certifications). Sometimes linking tables have logical names. Other times, they don't. In that case, I simply combine the names of the base tables.

Figure 2 shows how the rationalized relationship is diagramed.

Figure 2: Rationalized Employee-Certification Relationship

Now I can see where to put my unassigned Certification Date field. The Employee/Certification entity represents a certification for a particular employee and that can be given at only one time. Therefore the Certification Date field goes in this new entity. Figure 3 shows the completed Attribute Grid.

Figure 3: Final Attribute Grid

Now that I've got all the relationships between my entities identified and assigned all the attributes, I can put it all into one diagram.

Figure 4: Final E-R Diagram

Now I've got all the pieces. All that's left is to implement my diagram in Access (or some other database). In Entity-Relationship Diagramming: Part IV, I'll do just that.