Database Design for Mere Mortals: A Hands-On Guide to Relational Database Design

Reviewing the Initial Table Structures

Now that the fundamental table definitions are complete, you need to conduct interviews with users and management to review the work you've done so far. This set of interviews is fairly straightforward and should be relatively easy to conduct.

During these interviews, you will accomplish these tasks :

• Ensure that the appropriate subjects are represented in the database. Although it's highly unlikely that an important subject is missing at this stage of the database-design process, it can happen. When it does happen, identify the subject, use the proper techniques to transform it into a table, and develop it to the same degree as the other tables in the database.

• Make certain that the table names and table descriptions are suitable and meaningful to everyone. When a name or description appears to be confusing or ambiguous to several people in the organization, work with them to clarify the item as much as possible. It's common for some table names and descriptions to improve during the interview process.

• Make certain that the field names are suitable and meaningful to everyone. Selecting field names typically generates a great deal of discussion, especially when there is an existing database in place. You'll commonly find people who customarily refer to a particular field by a certain name because "that's what it's called on my screen." When you change a field nameyou have good reasons for doing soyou must diplomatically explain to these folks that you renamed the field so that it conforms to the standards imposed by the new database. You can also tell them that the field can appear with the more familiar name once the database is implemented in an RDBMS program. What you've said is true; many RDBMSs allow you to use one name for the field's physical definition and another name for display purposes. This feature, however, does not change, reduce, or negate the need for you to follow the guidelines for creating field names that you learned in Chapter 7.

• Verify that all the appropriate fields are assigned to each table. This is your best opportunity to make certain that all of the necessary characteristics pertaining to the subject of the table are in place. You'll commonly discover that you accidentally overlooked one or two characteristics earlier in the design process. When this happens, identify the characteristics, use the appropriate techniques to transform them into fields, and follow all the necessary steps to add them to the table.

When you've completed the interviews, you'll move to the next phase of the database-design process and establish field specifications for every field in the database.

CASE STUDY

It's now time to establish keys for each table in the Mike's Bikes database. As you know, your first order of business is to establish candidate keys for each table. Let's say you decide to start with the CUSTOMERS table in Figure 8.9.

As you review each field, you try to determine whether it conforms to the Elements of a Candidate Key. You determine that S TATUS , C UST H OME P HONE , and the combination of C UST F IRST N AME and C UST L AST N AME are potential candidate keys, but you're not quite certain whether any of them will completely conform to all of the elements. So you decide to test the keys by loading the table with sample data as shown in Figure 8.10.

Always remember that a field must comply with all of the Elements of a Candidate Key in order to qualify as a candidate key. You must immediately disqualify the field if it does not fulfill this requirement.

As you examine the table, you draw these conclusions:

• S TATUS is ineligible because it will probably contain duplicate values. As business grows, Mike is going to have many "Valued" customers.

• C UST H OME P HONE is ineligible because it will probably contain duplicate values. The sample data reveals that two customers can live in the same residence and have the same phone number.

• C UST F IRST N AME and C UST L AST N AME are ineligible because they will probably contain duplicate values. The sample data reveals that the combination of first name and last name can represent more than one distinct customer.

These findings convince you to establish an artificial candidate key for this table. You then create a field called C USTOMER ID, confirm that it complies with the requirements for a candidate key, and add the new field to the table structure with the appropriate designation.

Figure 8.11 shows the revised structure of the CUSTOMERS table.

Now you'll repeat this procedure for each table in the database. Remember to make certain that every table has at least one candidate key.

The next order of business is to establish a primary key for each table. As you know, you select the primary key for a particular table from the table's pool of available candidate keys. Here are a few points to keep in mind when you're choosing a primary key for a table with more than one candidate key:

• Choose a simple (single-field) candidate key over a composite candidate key.

• If possible, pick a candidate key that has the table name incorporated into its own name.

• Select the candidate key that best identifies the subject of the table or is most meaningful to everyone in the organization.

You begin by working with the EMPLOYEES table in Figure 8.12. As you review the candidate keys, you decide that E MPLOYEE N UMBER is a much better choice for a primary key than the combination of E MP F IRST N AME and E MP L AST N AME because Mike's employees are already accustomed to identifying themselves by their assigned numbers . Using E MPLOYEE N UMBER makes perfect sense, so you select it as the primary key for the table.

Now you perform one final task before you designate E MPLOYEE N UMBER as the official primary key of the table: You make absolutely certain that it exclusively identifies the value of each field within a given record. So, you test E MPLOYEE N UMBER by following these steps:

1. Load the EMPLOYEES table with sample data.

2. Select a record for test purposes and note the current value of E MPLOYEE N UMBER .

3. Examine the value of the first field (the one immediately after E MPLOYEE N UMBER ) and ask yourself this question:

   Does this primary key value exclusively identify the current value of <fieldname> ?

   1. If the answer is yes, move to the next field and repeat the question.

   2. If the answer is no, remove the field from the table , move to the next field and repeat the question. (Be sure to determine whether you can add the field you just removed to another table structure, if appropriate, or discard it completely because it is truly unnecessary.)

4. Continue this procedure until you've examined every field value in the record.

You know that you'll have to remove any field containing a value that E MPLOYEE N UMBER does not exclusively identify. E MPLOYEE N UMBER does exclusively identify the value of each field in the test record, however, so you use it as the official primary key for the EMPLOYEES table and mark its name with the letters "PK" in the table structure. You then repeat this process with the rest of the tables in Mike's new database until every table has a primary key.

Remember to keep these rules in mind as you establish primary keys for each table:

• Each table must have oneand only oneprimary key.

• Each primary key within the database should be uniqueno two tables should have the same primary key (unless one of them is a subset table).

As you work through the tables in Mike's database, you remember that the SERVICES table is a subset table. You created it during the previous stage of the design process (in Chapter 7), and it represents a more specific version of the subject represented by the PRODUCTS table. The P RODUCT N AME field is what currently relates the PRODUCTS table to the SERVICES subset table. You now know, however, that a subset table must have the same primary key as the table to which it is related , so you'll use P RODUCT N UMBER (the primary key of the PRODUCTS table) as the primary key of the SERVICES table. Figure 8.13 shows the PRODUCTS and SERVICES tables with their primary keys.

The last order of business is to conduct interviews with Mike and his staff and review all the work you've performed on the tables in the database. As you conduct these interviews, make certain you check the following:

• That the appropriate subjects are represented in the database

• That the table names and descriptions are suitable and meaningful to everyone

• That the field names are suitable and meaningful to everyone

• That all the appropriate fields are assigned to each table

By the end of the interview, everyone agrees that the tables are in good form and that all the subjects with which they are concerned are represented in the database. Only one minor point came up during the discussions: Mike wants to add a C ALL P RIORITY field to the VENDORS table. There are instances in which more than one vendor supplies a particular product, and Mike wants to create a way to indicate which vendor he should call first if that product is unexpectedly out of stock. So, you add the new field to the VENDORS table and bring the interview to a close.