Your next task is to establish keys for each table in the database. There are four main types of keys: candidate, primary, foreign, and non-keys. A key's type determines its function within the table.
The first type of key you establish for a table is the candidate key, which is a field or set of fields that uniquely identifies a single instance of the table's subject. Each table must have at least one candidate key. You'll eventually examine the table's pool of available candidate keys and designate one of them as the official primary key for the table.
Before you can designate a field as a candidate key, you must make certain it complies with all of the Elements of a Candidate Key. These elements constitute a set of guidelines you can use to determine whether the field is fit to serve as a candidate key. You cannot designate a field as a candidate key if it fails to conform to any of these elements.
It cannot be a multipart field. You've seen the problems with multipart fields, so you know that using one as an identifier is a bad idea.
It must contain unique values. This element helps you guard against duplicating a given record within the table. Duplicate records are just as bad as duplicate fields, and you must avoid them at all costs.
It cannot contain null values. As you already know, a null value represents the absence of a value. There's absolutely no way a candidate key field can identify a given record if its value is null.
Its value cannot cause a breach of the organization's security or privacy rules. Values such as passwords and Social Security Numbers are not suitable for use as a candidate key.
Its value is not optional in whole or in part. A value that is optional implies that it may be null at some point. You can infer, then, that an optional value automatically violates the previous element and is, therefore, unacceptable. (This caveat is especially applicable when you want to use two or more fields as a candidate key.)
It comprises a minimum number of fields necessary to define uniqueness. You can use a combination of fields (treated as a single unit) to serve as a candidate key, so long as each field contributes to defining a unique value. Try to use as few fields as possible, however, because overly complex candidate keys can ultimately prove to be difficult to work with and difficult to understand.
Its values must uniquely and exclusively identify each record in the table. This element helps you guard against duplicate records and ensures that you can accurately reference any of the table's records from other tables in the database.
Its value must exclusively identify the value of each field within a given record. This element ensures that the table's candidate keys provide the only means of identifying each field value within the record. (You'll learn more about this particular element in the section on primary keys.)
Its value can be modified only in rare or extreme cases. You should never change the value of a candidate key unless you have an absolute and compelling reason to do so. A field is likely to have difficulty conforming to the previous elements if you can change its value arbitrarily.
Establishing a candidate key for a table is quite simple: Look for a field or set of fields that conforms to all of the Elements of a Candidate Key. You'll probably be able to define more than one candidate key for a given table. Loading a table with sample data will give you the means to identify potential candidate keys accurately. (You used this same technique in the previous chapter.)
See if you can identify any candidate keys for the table in Figure 8.1.
You probably identified EMPLOYEE ID, SOCIAL SECURITY NUMBER, EMPLAST NAME, EMPFIRST NAME and EMPLAST NAME, EMPZIPCODE, and EMPHOME PHONE as potential candidate keys. But you'll need to examine these fields more closely to determine which ones are truly eligible to become candidate keys. Remember that you must automatically disregard any field(s) failing to conform to even one of the Elements of a Candidate Key.
Upon close examination, you can draw the following conclusions:
EMPLOYEE ID is eligible. This field conforms to every element of a candidate key.
SOCIAL SECURITY NUMBER is ineligible because it could contain null values and will most likely compromise the organization's privacy rules. Contrary to what the sample data shows, this field could contain a null value. For example, there are many people working in the United States who do not have Social Security numbers because they are citizens of other countries.
Despite its widespread use in many types of databases, I would strongly recommend that you refrain from using SOCIAL SECURITY NUMBER as a candidate key (or a primary key, for that matter) in any of your database structures. In many instances, it doesn't conform to the Elements of a Candidate Key. You can learn some very interesting facts about Social Security numbers (which will shed some light on why they make poor candidate/primary keys) by visiting the Social Security Adminstration's Web site at http://www.ssa.gov.
EMPLAST NAME is ineligible because it can contain duplicate values. As you've learned, the values of a candidate key must be unique. In this case there can be more than one occurrence of a particular last name.
EMPFIRST NAME and EMPLAST NAME are eligible. The combined values of both fields will supply a unique identifier for a given record. Although multiple occurrences of a particular first name or last name will occur, the combination of a given first name and last name will always be unique. (Some of you are probably saying, "This is not necessarily always true." You're absolutely right. Don't worry; we'll address this issue shortly.)
EMPZIPCODE is ineligible because it can contain duplicate values. Many people live in the same zip code area, so the values in EMPZIPCODE cannot possibly be unique.
EMPHOME PHONE is ineligible because it can contain duplicate values and is subject to change. This field will contain duplicate values for either of these reasons:
One or more family members work for the organization.
One or more people share a residence that contains a single phone line.
You can confidently state that the EMPLOYEES table has two candidate keys: EMPLOYEE ID and the combination of EMPFIRST NAME and EMPLAST NAME.
Mark candidate keys in your table structures by writing the letters "CK" next to the name of each field you designate as a candidate key. A candidate key composed of two or more fields is known as a composite candidate key, and you'll write "CCK" next to the names of the fields that make up the key. When you have two or more composite candidate keys, use a number within the mark to distinguish one from another. If you had two composite candidate keys, for example, you would mark one as "CCK1" and the other as "CCK2."
Apply this technique to the candidate keys for the EMPLOYEES table in Figure 8.1. Figure 8.2 shows how your structure should look when you've completed this task.
Now, try to identify as many candidate keys as you can for the PARTS table in Figure 8.3.
At first glance, you may believe that PART NAME, MODEL NUMBER, the combination of PART NAME and MODEL NUMBER, and the combination of MANUFACTURER and PART NAME are potential candidate keys. After investigating this theory, however, you come up with the following results:
PART NAME is ineligible because it can contain duplicate values. A given part name will be duplicated when the part is manufactured in several models. For example, this is the case with Faust Brake Levers.
MODEL NUMBER is ineligible because it can contain null values. A candidate key value must exist for each record in the table. As you can see, some parts do not have a model number.
PART NAME and MODEL NUMBER are ineligible because either field can contain null values. The simple fact that MODEL NUMBER can contain null values instantly disqualifies this combination of fields.
MANUFACTURER and PART NAME are ineligible because the values for these fields seem to be optional. Recall that a candidate key value cannot be optional in whole or in part. In this instance, you can infer that entering the manufacturer name is optional when it appears as a component of the part name; therefore, you cannot designate this combination of fields as a candidate key.
It's evident that you don't have a single field or set of fields that qualifies as a candidate key for the PARTS table. This is a problem because each table must have at least one candidate key. Fortunately, there is a solution.
When you determine that a table does not contain a candidate key, you can create and use an artificial (or surrogate) candidate key. (It's artificial in the sense that it didn't occur "naturally" in the table; you have to manufacture it.) You establish an artificial candidate key by creating a new field that conforms to all of the Elements of a Candidate Key and then adding it to the table; this field becomes the official candidate key.
You can now solve the problem in the PARTS table. Create an artificial candidate key called PART NUMBER and assign it to the table. (The new field will automatically conform to the Elements of a Candidate Key because you're creating it from scratch.) Figure 8.4 shows the revised structure of the PARTS table.
When you've established an artificial candidate key for a table, mark the field name with a "CK" in the table structure, just as you did for the EMPLOYEES table in the previous example.
You may also choose to create an artificial candidate key when it would be a stronger (and thus, more appropriate) candidate key than any of the existing candidate keys. Assume you're working on an EMPLOYEES table and you determine that the only available candidate key is the combination of the EMPFIRST NAME and EMPLAST NAME fields. Although this may be a valid candidate key, using a single-field candidate key might prove more efficient and may identify the subject of the table more easily. Let's say that everyone in the organization is accustomed to using a unique identification number rather than a name as a means of identifying an employee. In this instance, you can choose to create a new field named EMPLOYEE ID and use it as an artificial candidate key. This is an absolutely acceptable practicedo this without hesitation or reservation if you believe it's appropriate.
I commonly create an ID field (such as EMPLOYEE ID, VENDOR ID, DEPARTMENT ID, CATEGORY ID, and so on) and use it as an artificial candidate key. It always conforms to the Elements of a Candidate Key, makes a great primary key (eventually), and, as you'll see in Chapter 10, makes the process of establishing table relationships much easier.
Review the candidate keys you've selected and make absolutely certain that they thoroughly comply with the Elements of a Candidate Key. Don't be surprised if you discover that one of them is not a candidate key after allincorrectly identifying a field as a candidate key happens occasionally. When this does occur, just remove the "CK" designator from the field name in the table structure. Deleting a candidate key won't pose a problem as long as the table has more than one candidate key. If you discover, however, that the only candidate key you identified for the table is not a candidate key, you must establish an artificial candidate key for the table. After you've defined the new candidate key, remember to mark its name with a "CK" in the table structure.
By now, you've established all the candidate keys that seem appropriate for every table. Your next task is to establish a primary key for each table, which is the most important key of all.
A primary key field exclusively identifies the table throughout the database structure and helps establish relationships with other tables. (You'll learn more about this in Chapter 10.)
A primary key value uniquely identifies a given record within a table and exclusively represents that record throughout the entire database. It also helps to guard against duplicate records.
A primary key must conform to the exact same elements as a candidate key. This requirement is easy to fulfill because you select a primary key from a table's pool of available candidate keys. The process of selecting a primary key is somewhat similar to that of a presidential election. Every four years, several people run for the office of president of the United States. These individuals are known as "candidates" and they have all of the qualifications required to become president. A national election is held, and a single individual from the pool of available presidential candidates is elected to serve as the country's official president. Similarly, you identify each qualified candidate key in the table, run your own election, and select one of them to become the official primary key of the table. You've already identified the candidates, so now it's election time!
Assuming that there is no other marginal preference, here are a couple of guidelines you can use to select an appropriate primary key:
If you have a simple (single-field) candidate key and a composite candidate key, choose the simple candidate key. It's always best to use a candidate key that contains the least number of fields.
Choose a candidate key that incorporates part of the table name within its own name. For example, a candidate key with a name such as SALES INVOICE NUMBER is a good choice for the SALES INVOICES table.
Examine the candidate keys and choose one to serve as the primary key for the table. The choice is largely arbitraryyou can choose the one that you believe most accurately identifies the table's subject or the one that is the most meaningful to everyone in the organization. For example, consider the EMPLOYEES table again in Figure 8.5.
Either of the candidate keys you identified within the table could serve as the primary key. You might decide to choose EMPLOYEE ID if everyone in the organization is accustomed to using this number as a means of identifying employees in items such as tax forms and employee benefits programs. The candidate key you ultimately choose becomes the primary key of the table and is governed by the Elements of a Primary Key. These elements are exactly the same as those for the candidate key, and you should enforce them to the letter. For the sake of clarity, here are the Elements of a Primary Key:
It cannot be a multipart field.
It must contain unique values.
It cannot contain null values.
Its value cannot cause a breach of the organization's security or privacy rules.
Its value is not optional in whole or in part.
It comprises a minimum number of fields necessary to define uniqueness.
Its values must uniquely and exclusively identify each record in the table.
Its value must exclusively identify the value of each field within a given record.
Its value can be modified only in rare or extreme cases.
Before you finalize your selection of a primary key, it is imperative that you make absolutely certain that the primary key fully complies with this particular element:
Its value must exclusively identify the value of each field within a given record.
Each field value in a given record should be unique throughout the entire database (unless it is participating in establishing a relationship between a pair of tables) and should have only one exclusive means of identificationthe specific primary key value for that record.
You can determine whether a primary key fully complies with this element by following these steps:
Load the table with sample data.
Select a record for test purposes and note the current primary key value.
Examine the value of the first field (the one immediately after the primary key) and ask yourself this question:
Does this primary key value exclusively identify the current value of <fieldname>?
If the answer is yes, move to the next field and repeat the question.
If the answer is no, remove the field from the table, move to the next field and repeat the question.
Continue this procedure until you've examined every field value in the record.
A field value that the primary key does not exclusively identify indicates that the field itself is unnecessary to the table's structure; therefore, you should remove the field and reconfirm that the table complies with the Elements of the Ideal Table. You can then add the field you just removed to another table structure, if appropriate, or you can discard it completely because it is truly unnecessary.
Here's an example of how you might apply this technique to the partial table structure in Figure 8.6. (Note that INVOICE NUMBER is the primary key of the table.)
First, you load the table with sample data. You then select a record for test purposeswe'll use the third record for this exampleand note the value of the primary key (13002). Now, pose the question above for each field value in the record.
Does this primary key value exclusively identify the current value of . . .
After you've removed the unnecessary fields you identified during this test, examine the revised table structure and make sure it complies with the Elements of the Ideal Table.
The primary key should now exclusively identify the values of the remaining fields in the table. This means that the primary key is truly sound and you can designate it as the official primary key for the table. Remove the "CK" next to the field name in the table structure and replace it with a "PK." (A primary key composed of two or more fields is known as a composite primary key, and you mark it with the letters "CPK.") Figure 8.7 shows the revised structure of the SALES INVOICE table with INVOICE NUMBER as its primary key.
As you create a primary key for each table in the database, keep these two rules in mind:
Each table must have oneand only oneprimary key. Because the primary key must conform to each of the elements that govern it, only one primary key is necessary for a particular table.
Each primary key within the database must be uniqueno two tables should have the same primary key unless one of them is a subset table. You learned at the beginning of this section that the primary key exclusively identifies a table throughout the database structure; therefore, each table must have its own unique primary key in order to avoid any possible confusion or ambiguity concerning the table's identity. A subset table is excluded from this rule because it represents a more specific version of a particular data table's subjectboth tables must share the same primary key.
Later in the database-design process, you'll learn how to use the primary key to help establish a relationship between a pair of tables.
Now that you've selected a candidate key to serve as the primary key for a particular table, you'll designate the remaining candidate keys as alternate keys. These keys can be useful to you in an RDBMS program because they provide an alternative means of uniquely identifying a particular record within the table. If you choose to use an alternate key in this manner, mark its name with "AK" or "CAK" (composite alternate key) in the table structure; otherwise, remove its designation as an alternate key and simply return it to the status of a normal field. You won't be concerned with alternate keys for the remainder of the database-design process, but you will work with them once again as you implement the database in an RDBMS program. (Implementing and using alternate keys in RDBMS programs is beyond the scope of this workour only objective here is to designate them as appropriate. This is in line with the focus of the book, which is the logical design of a database.)
Figure 8.8 shows the final structure for the EMPLOYEES table with the proper designation for both the primary key and the alternate keys.
A non-key is a field that does not serve as a candidate, primary, alternate, or foreign key. Its sole purpose is to represent a characteristic of the table's subject, and its value is determined by the primary key. There is no particular designation for a non-key, so you don't need to mark it in the table structure.