Preventing Form Errors by Validating Data

Preventing Form Errors by Validating Data

If, as the cooks say, a recipe is only as good as its ingredients , a database is only as good as its data. Viewing, summarizing, and analyzing the data are meaningless if the table you're working with contains erroneous or improper data. For basic data errors (for example, entering the wrong date or transposing a number's digits), there's not a lot you can do other than exhorting yourself or the people who use your forms to enter data carefully . Fortunately, you have a bit more control when it comes to preventing improper data entry. By "improper," I mean data that falls into either of the following categories:

• Data that is the wrong type. For example, entering a text string in a cell that requires a number.

• Data that falls outside of an allowable range. For example, entering 200 in a cell that requires a number between 1 and 100.

The next few sections show you several techniques that can help you reduce these types of errors.

Helping Users with Text Prompts

You can prevent improper entries to a certain extent by adding text that provides details on what is allowable inside a particular cell. You have two choices:

• Add status bar text This is a string that appears in the Access status bar when users enter the field. You specify this text by opening the field's property sheet, displaying the Other tab, and then entering the string in the Status Bar Text property.

• Add a label Place a Label control near the field and use it to enter text that describes the field's data requirements or shortcut keys. For example, if the field requires a date, the label might say Press Ctrl+; to enter today's date .

For instance, Figure 4.25 shows a form used as a Mortgage Calculator. Notice the labels added beside the Interest Rate and Term text boxes that specify to the users that they must enter the interest rate per annum and the term in years . Note, too, the status bar text that appears when the users enter the Interest Rate field.

Preventing Errors with Data Validation Expressions

The problem with text prompts is they require other people to both read and act on the text. The better solution for preventing data-entry errors is the Access data validation feature. With data validation, you create rules that specify exactly what kind of data can be entered and in what range that data can fall. You can also specify pop-up input messages that appear when a cell is selected, as well as error messages that appear when data is entered improperly.

Follow these steps to define the settings for a data validation rule:

For example, suppose you want the users to enter an interest rate. This should be a positive quantity, of course, but it should also be less than 1. (That is, you want to users to enter 6% as 0.06 instead of 6.) Figure 4.26 shows the property sheet for a field named InterestRate that meets these criteria by defining the following expression in the Validation Rule property:

>0 And <1

Figure 4.26 also shows a string in the Validation Text property. If the users enter invalid data (that is, any value for which the Validation Rule expression returns False), the Validation Text appears in a dialog box, as shown in Figure 4.27.

Using Input Masks for Consistent and Accurate Data Entry

One of the major headaches that database administrators have to deal with is data entered in an inconsistent way. For example, consider the following phone numbers :

(123)555-6789
(123) 555-6789
(123)5556789
123555-6789
1235556789

These kinds of inconsistencies might appear trivial, but they can cause all kinds of problems, from other users misreading the data, to improper sorting, to difficulties analyzing or querying the data. And it isn't just phone numbers that cause problems. You also see them with social security numbers, ZIP Codes, dates, times, account numbers, and more.

One way to avoid such inconsistencies is to add a label or status bar message that specifies the correct format to use. As with data validation, however, these prompts are not guaranteed to work every time (or even most of the time).

A better solution is to apply an input mask to the field. An input mask is a kind of template that shows users how to enter the data and prevents them from entering incorrect characters (such as a letter where a number is required). For example, here's an input mask for a phone number:

(___)___-____

Each underscore (_) acts as a placeholder for (in this case) a digit, and the parentheses and dash appear automatically as the user enters the number.

Using the Input Mask Wizard

The easiest way to create an input mask is to use the Input Mask Wizard. Here are the steps to follow:

Creating a Custom Input Mask Expression

If your data doesn't fit any of the predefined input masks, you need to create a custom mask that suits your needs. You do this by creating an expression that consists of three kinds of characters:

• Data placeholders These characters are replaced by the actual data typed by the users. The different placeholders specify the type of character the users must enter (such as a digit or letter) and whether the character is optional or required.

• Modifiers These characters aren't displayed in the mask; instead, they're used to modify the mask in some way (such as converting all the entered characters to lowercase).

• Literals These are extra characters that appear in the mask the same as you enter them in the expression. For example, you might use parentheses as literals to surround the area code portion of a phone number.

Table 4.3 lists the data placeholders you can use to build your input mask expressions.

Table 4.3. Data Placeholders to Use for Custom Input Masks

Table 4.4 lists the modifiers and literals you can use to build your input mask expressions.

Table 4.4. Modifiers and Literals to Use for Custom Input Masks

>aaaa\-0000

first;second;third

00000\-9999;0;.