Even the best database designer has spent a sleepless night worrying about the errors that could be lurking in a database. Bad dataâs a notorious problemâit enters the database, lies dormant for months, and appears only when you discover youâve mailed an invoice to customer âBlank Blankâ or sold a bag of peanuts forâ$4.99.
The best way to prevent these types of problems is to stop bad data from making it into your database in the first place. In other words, you need to set up validation rules that reject suspicious values as soon as someone types them in. Once bad dataâs entered your database, itâs harder to spot than a blueberry in a swimming pool.
This chapter covers the essential set of Access data validation tools:
The basics include duplicates, required fields, and default values.
Input masks format ordinary text into patterns, like postal codes and phone numbers.
Validation rules lay down strict laws for unruly fields.
Lookups limit values to a list of preset choices.
All of Accessâs data validation features work via the Design view you learned about in Chapter 2. To put them in place, you choose a field and then tweak its properties. The only trickâs knowing what properties are most useful. Youâve already seen some in Chapter 2, but the following sections fill in a few more details.
Tip
Remember, Access gives you three ways to switch to Design view. You can right-click the table tab title and then choose Design View from the menu, use the Home â View button on the ribbon, or use the tiny view buttons at the Access windowâs bottom-right corner. And if youâre really impatient, then you donât even need to open your table firstâjust find it in the navigation pane, right-click it there, and then choose Design View.
Every record needs a bare minimum of information to make sense. However, without your help, Access canât distinguish between critical information and optional details. For that reason, every field in a new table is optional, except for the primary-key field (which is usually the ID value). Try this out with the Dolls table from Chapter 1; youâll quickly discover that you can add records that have virtually no information in them.
You can easily remedy this problem. Just select the field that you want to make mandatory in Design view, and then set the Required field property to Yes (Figure 4-1).
Figure 4-1. The Required field property tells Access not to allow empty values (called nulls in tech-speak).
Access checks the Required field property whenever you add a new record or modify a field in an existing record. However, if your table already contains data, thereâs no guarantee that it follows the rules.
Imagine you fill the Dolls table with a few bobbleheads before you decide that every record requires a value for the Character field. You switch to Design view, choose the Character field, and then flip the Required field property to Yes. When you save the table (by switching back to Datasheet view or closing the table), Access gives you the option of verifying the bobblehead records that are already in the table (Figure 4-2). If you choose to perform the test and Access finds the problem, it gives you the option of reversing your changes (Figure 4-3).
Figure 4-2. Itâs a good idea to test the data in your table to make sure it meets the new requirements you put into place. Otherwise, invalid data could still remain. Donât let the message scare youâunless you have tens of thousands of records, this check doesnât take long.
Figure 4-3. If Access finds an empty value, then it stops the search and asks you what to do about it. You can keep your changes (even though they conflict with at least one record)âafter all, at least new records wonât suffer from the same problem. Your other option is to reset your field to its more lenient previous self. Either way, you can track down the missing data by performing a sort on the field in question (Section 3.2), which brings empty values to the top.
Access supports this Required property for every data type. However, with some data types you might want to add extra checks. Thatâs because the Required property prevents only blank fieldsâfields that donât have any information in them at all. However, Access makes a slightly bizarre distinction between blank values and something called empty text.
A blank (null) value indicates that no information was supplied. Empty text indicates that a field value was supplied, but it just happens to be empty. Confused yet? The distinction exists because databases like Access need to recognize when informationâs missing. A blank value could indicate an oversightâsomeone might just have forgotten to enter the value. On the other hand, empty text indicates a conscious decision to leave that information out.
Note
To try this out in your datasheet, create a text field that has Required set to Yes. Try inserting a new record, and leaving the record blank. (Access stops you cold.) Now, try adding a new record, but place a single space in the field. Hereâs the strange part: Access automatically trims out the spaces, and by doing so, it converts your single space to empty text. However, you donât receive an error message because empty text isnât the same as a blank value.
The good news is that if you find this whole distinction confusing, then you can prevent both blank values and empty text. Just set Required to Yes to stop the blank values, and set Allow Zero Length to No to prevent empty text.
Note
A similar distinction exists for numeric data types. Even if you set Required to Yes, you can still supply a number of 0. If you want to prevent that action, then youâll need to use the validation rules described later in this chapter (Section 4.3).
So far, the fields in your tables are either filled in explicitly by the person who adds the record or left blank. But thereâs another optionâyou can supply a default value. Now, if someone inserts a record and leaves the field blank, Access applies the default value instead.
You set a default value using the Default Value field property. For a numeric AddedCost field, you could set this to be the number 0. For a text Country field, you could use the text âU.S.A.â as a default value. (All text values must be wrapped in quotations marks when you use them for a default value.)
Access shows all your default values in the new-row slot at the bottom of the datasheet (Figure 4-4). It also automatically inserts default values into any hidden columns (Section 3.1.4).
Access inserts the default value when you create a new record. (Youâre then free to change that value.) You can also switch a field back to its default value using the Ctrl+Alt+Space shortcut while youâre editing it.
Figure 4-4. This dating service uses four default values: a default height (5.9), a default city (New York), a default state (also New York), and a default country (U.S.A.). This system makes sense, because most of their new entries have this information. On the other hand, thereâs no point in supplying a default value for the name fields.
Tip
One nice feature is that you can use the default value as a starting point for a new record. For example, when you create a new record in the datasheet, you can edit the default value, rather than replacing it with a completely new value.
You can also create more intelligent dynamic default values. Access evaluates dynamic default values whenever you insert a new record, which means that the default value can vary based on other information. Dynamic default values use expressions (specialized database formulas) that can perform calculations or retrieve other details. One useful expression, Date(), grabs the current date thatâs set on your computer. If you use Date( ) as the default value for a date field (as shown in Figure 4-5), then Access automatically inserts the current date whenever you add a new record.
Note
Youâll learn much more about SQL expressions in Part Two.
In a properly designed table, every record must be unique. To enforce this restriction, you shoud choose a primary key (Section 2.4), which is one or more fields that wonât be duplicated.
Hereâs the catch. As you learned in Chapter 2, the safest optionâs to create an ID field for the primary key. So far, all the tables youâve seen have included this detail. But what if you need to make sure other fields are unique? Imagine you create an Employees table. You follow good database design principles and identify every record with an automatically generated ID number. However, you also want to make sure that no two employees have the same Social Security number (SSN) to prevent possible errorsâlike accidentally entering the same employee twice.
Figure 4-5. If you use the Date( ) function as the default value for the DateAcquired field in the bobblehead table, then every time you add a new bobblehead record, Access fills in the current date. You decide whether you want to keep that date or replace it with a different value.
Note
For a quick refresher about why ID fields are such a good idea, refer to Section 2.4.1. In the Employees table, you certainly could choose to make the SSN the primary key, but itâs not the ideal situation when you start linking tables together (Chapter 5), and it causes problems if you need to change the SSN later on (in the case of an error), or if you enter employee information before youâve received the SSN.
You can force a field to require unique values with an index. A database index is analogous to the index in a bookâitâs a list of values (from a field) with a cross-reference that points to the corresponding section (the full record). If you index the SocialSecurityNumber field, Access creates a list like this and stores it behind the scenes in your database file:
Table 4-1.Â
|
SocialSecurityNumber |
Location of Full Record |
|---|---|
|
001-01-3455 |
⦠|
|
001-02-0434 |
⦠|
|
001-02-9558 |
⦠|
|
002-40-3200 |
⦠|
Using this list, Access can quickly determine whether a new record duplicates an existing SSN. If it does, then Access doesnât let you insert it.
So how do you apply an index to a field? The trickâs the Indexed field property, which is available for every data type except Attachment and OLE Object. When you add a field, the Indexed propertyâs set to No, which means Access doesnât create an index. To add an index and prevent duplicates, you can change the Indexed property in Design view to Yes [No Duplicates]. The third option, Yes [Duplicates OK], creates an index but lets more than one record have the same value. This option doesnât help you catch repeated records, but you can use it to speed up searches (see the box, âHow Indexes Speed Up Searchesâ in Section 6.2.2 for more).
Note
As you know from Chapter 2 (Section 2.4.1), primary keys also disallow duplicates, using the same technique. When you define a primary key, Access automatically creates an index on that field.
When you close Design view after changing the Indexed field property, Access prompts you to save your changes. At this point, it creates any new indexes it needs. You canât create a no-duplicates index if you already have duplicate information in your table. In this situation, Access gives you an error message when you close the Design window and it attempts to add the index.
You can also use indexes to prevent a combination of values from being repeated. Imagine you create a People table to track your friends and their contact information. Youâre likely to have entries with the same first or last name. However, you may want to prevent two records from having the same first and last name. This limitation prevents you from inadvertently adding the same person twice.
Note
This example could cause endless headaches if you honestly do have two friends who share the same first and last names. In that case, youâll need to remove the index before youâre allowed to add the name. You should think carefully about legitimate reasons for duplication before you create any indexes.
To ensure that a combination of fields is unique, you need to create a compound index, which combines the information from more than one field. Hereâs how to do it:
In Design view, choose Table Tools | Design â Show/Hide â Indexes.
The Indexes window appears (Figure 4-6). Using the Indexes window, you can see your current indexes and add new ones.
Choose a name for your index. Type this name into the first blank row in the Index Name column.
The index name has no real importanceâAccess uses it to store the index in the database, but you donât see the index name when you work with the table. Usually, youâll use the name of one or both of the fields youâre indexing (like Last-Name+FirstName).
Choose the first field in the Field Name column in the same row (like Last-Name).
It doesnât matter which field name you use first. Either way, the index can prevent duplicate values. However, the order does affect how searches use the index to boost performance. Youâll learn more in Section 6.2.2.
In the area at the bottom of the window, set the Unique box to Yes.
This creates an index that prevents duplicates (as opposed to one thatâs used only for boosting search speeds).
You can also set the Ignore Nulls box to Yes, if you want Access to allow duplicate blank values. Imagine you want to make the SSN field optional. However, if an SSN number is entered, then you want to make sure it doesnât duplicate any other value. In this case, you should set Ignore Nulls to Yes. If you set Ignore Nulls to No, then Access lets only one record have a blank SSN field, which probably isnât the behavior you want.
Note
You can also disallow blank values altogether using the Required property, as described in Section 4.1.1
Ignore the Primary box (which identifies the index used for the primary key).
Move down one row. Leave the Index Name column blank (which tells Access itâs still part of the previous index), but choose another field in the Field Name column (like FirstName).
If you want to create a compound index with more than two fields, then just repeat this step until youâve added all the fields you need. Figure 4-7 shows what a finished index looks like.
As youâve already learned, databases prize consistency. If you have a field named Height, you better be sure every value in that field uses the same type of measurements; otherwise, your dataâs not worth its weight in sock lint. Similarly, if you have a PhoneNumber field, you better make sure every phone number has the same format. If some phone numbers are written with dashes, spaces, and parentheses (like (844) 547-1123), while others are a bit different (say 847-547-1123), and a few leave out the area code information altogether (547-1123), then youâve got a small problem on your hands. Because of the lack of consistency, youâll have a hard time working with this information (say, searching for a specific phone number or sorting the phone numbers into different categories based on area code).
To help you manage values that have a fixed patternâlike phone numbersâyou can use an input mask. Essentially, an input mask (or just mask for short) gives you a way to tell Access what pattern your data should use. Based on this pattern, Access changes the way values are entered and edited to make them easier to understand and less error-prone. Figure 4-8 shows how a mask lets Access format a series of characters as theyâre being typed into a field.
Figure 4-8. Top: Hereâs a PhoneNumber field with a mask thatâs ready to go. So far, the person entering the record hasnât typed anything. The PhoneNumber field automatically starts out with this placeholder text. Bottom: The mask formats the numbers as you type. If you type 1234567890 into this phone number mask, then you see the text (123) 456-7890. Behind the scenes, the databases stores 1234567890, but the informationâs presented in the datasheet using a nicely formatted package. That package is the mask.
You can add a mask to any field that uses the Text data type. Masks give you several advantages over ordinary text:
Masks guide data entry. When empty, a masked edit control shows the place-holders where values need to go. A phone number mask shows the text (_ _ _) _ _ _-_ _ _ _ when itâs empty, clearly indicating what type of information it needs.
Masks make data easier to understand. You can read many values more easily when theyâre presented a certain way. Most people can pick out the numbers in this formatted Social Security number (012-86-7180) faster than this unformatted one (012867180).
Masks prevent errors. Masks reject characters that donât fit the mold. If youâre using the telephone mask, you canât use letters.
Masks prevent confusion. With many types of data, you have several ways to present the same information. You can enter phone numbers both with and without area codes. By presenting the mask with the area code placeholder, youâre saying that this informationâs required (and where it goes). Itâs also obvious that you donât need to type in parentheses or a dash to separate numbers, because those details are already there. Youâll see the same benefit if you use masks with dates, which can be entered in all sorts of different combinations (Year/Month/Day, Month-Day-Year, and so on).
Masks are best suited for when youâre storing numeric information in a text field. This scenario occurs with all sorts of data, including credit card numbers, postal codes, and phone numbers. These types of information shouldnât be stored in number fields, because they arenât meant to be interpreted as a single number. Instead, theyâre meant to be understood as a series of digits. (If you do make the mistake of storing a phone number in a number field, youâll find out that people can type in perfectly nonsensical phone numbers like 0 andâ14 because these are valid numbers, even if they arenât valid phone numbers. But an input mask on a text field catches these errors easily.)
Masks canât help you with more sophisticated challenges, like data values that have varying lengths or subtle patterns. For instance, a mask doesnât help you spot an incorrect email address.
The easiest way to get started with masks is to use one of the many attractive options that Access has ready for you. This methodâs great, because it means you donât need to learn the arcane art of mask creation.
Hereâs what you need to do to pick out a prebuilt mask:
In Design view, select the text field where you want to apply the mask.
For this test, try a PhoneNumber field.
Look for the Input Mask field property. Click inside the field.
When you do, a small ellipsis (â¦) button appears at the left edge, as shown in Figure 4-9.
Click the ellipsis button.
The Input Mask wizard starts (see Figure 4-10).
Figure 4-10. The Input Mask wizard starts with a short list of commonly used masks. Next to every mask, Access shows you what a sample formatted value looks like. Once you select a mask, you can try using it in the Try It text box. The Try It text box gives you the same behavior that your field will have once you apply the mask.
Choose the mask you want from the list of options.
In this case, choose the first item in the list (Phone Number).
Note
Donât see what you want? Youâll need to create your own, using the tips in Section 4.2.2. If you see one thatâs close but not perfect, select it. You can tweak the mask in the wizardâs second step.
Click Next.
The wizardâs second step appears (see Figure 4-11).
If you want, you can change the mask or the placeholder character.
To change the mask, youâll need to learn what every mask character means. Section 4.2.2 explains it all.
Figure 4-11. The phone number mask is !(999) 000-000. Each 9 represents an optional number from 0 to 9. Each 0 represents a required number from 0 to 9. So according to this mask, (123) 456-7890 is a valid phone number, as is 123-4567, but (123) 456 isnât.
You use the placeholder to show the empty slots where you enter information. The standard choice is the underscore. Optionally, you can use a space, dash, asterisk, or any other character by typing it in the âPlaceholder characterâ box.
Click Next.
If youâre adding a mask to a text field, then the wizardâs final step appears (see Figure 4-12).
If youâre adding a mask to a date field, then Access doesnât need to ask you how to store the informationâit already knows. In this case, you can jump to step 9 and click Finish.
Choose how you want to store the value in this field.
The standard choice is to store just the characters youâve typed in (in other words, everything you type into the field). If you use this option, the placeholders arenât included. For example, the phone number (416) 123-4567 is stored as 4161234567. This option saves a little space, and it also lets you change the mask later on to present the information in a slightly different way.
You could also store the mask complete with all the extra characters. Then a phone numberâs stored complete with hyphens, dashes, and spaces, like (416) 123-4567. This approach isnât nearly as flexible because you canât change the mask later.
Click Finish.
The final mask appears in the Input Mask field property.
Before going any further, you may want to make sure that the length youâve reserved for your field matches the mask. In the phone number example, you need a Field Size of 10 if youâve chosen to store unformatted values (because there are 10 digits), or a Field Size of 14 for the whole shebang, complete with placeholders (one dash, one space, and two parentheses).
Switch back to the Datasheet view, and click Yes when Access asks you to save changes.
Your input mask is now in place.
Note
Access uses the input mask information to control how you enter information in the datasheet. However, itâs possible to circumvent the mask by entering the information in other ways. You could, for instance, create a form (as described in Part Four), and switch off the mask. A maskâs not an absolute guarantee against invalid dataâif you want such a guarantee, then you need a validation rule instead.
The Input Mask wizard provides a fairly limited set of choices. If you want to use a mask with your own type of information (like a special customer code that your business uses), then youâll have to create your own mask.
Creating a maskâs fairly easy, but it can take a bit of fiddling before you nail down exactly the result you want. You have two basic options:
Type or edit the mask directly in the Input Mask field property.
Launch the Input Mask wizard, choose a mask to use as a starting point (as described in Section 4.2.1), and then tweak it in step 2. This approach has the advantage that you can test your mask in the Try It box before you save it as part of your table.
Every maskâs built out of three types of characters:
Placeholders designate where you type in a character.
Special characters give additional instructions that tell Access how to treat a part of the mask.
Literals are all other characters, which are really just decoration to help make the value easier to interpret.
In the previous example, the phone number mask was !(999) 000-000. The characters 9 and 0 are placeholdersâthey represent where you type in the digits of the phone number. The parentheses, space, and dash are just formatting nicetiesâtheyâre the literals. And the exclamation markâs the only special character. It tells Access that characters should be entered into the mask from left to right, which is the standard option and the only one that really makes sense for a phone number.
To help you sort all this out, refer to the following tables. Table 4-1 shows all the placeholders you can use in an input mask. Table 4-2 shows other special characters. Everything else is automatically a literal character.
Table 4-2. Placeholder Characters for an Input Mask
|
Character |
Description |
|---|---|
|
0 |
A required digit (0 through 9). |
|
9 |
An optional digit (0 through 9). |
|
# |
An optional digit, a plus sign (+), or a minus sign (â) |
|
L |
A required letter. |
|
? |
An optional letter. |
|
A |
A required letter or digit. |
|
a |
An optional letter or digit. |
|
& |
A required character of any type (including letters, numbers, punctuation, and so on). |
|
C |
An optional character of any type (including letters, numbers, punctuation, and so on). |
Table 4-3. Special Characters for an Input Mask
|
Character |
Description |
|---|---|
|
! |
Indicates that the maskâs filled from left to right when characters are typed in. This is the default, so this characterâs not required (although the prebuilt masks include it). |
|
< |
Converts all characters that follow to lowercase. |
|
> |
Converts all characters that follow to uppercase. |
|
\ |
Indicates that the following character should be treated as a literal. For example, the # character has a special meaning in masks. Thus, if you want to actually include a # in your mask, you need to use \#. Sometimes, this characterâs used before a placeholder even when itâs not needed. You may see a phone mask that has the character sequence \- instead of just -. Both are equivalent. |
|
Password |
Creates a password entry box. Any character you type in the box is stored as the character but displayed as an asterisk (*). When using this option, you canât include anything else in your mask. |
Here are a few sample masks to get you started:
(000) 000-000. A phone number that requires the area code digits. This maskâs different from the phone number mask that the Input Mask wizard uses. That mask replaces the first three 0 characters with 9, making the area code optional.
00000-9999. A U.S. zip code, which consists of five required digits followed by a hyphen and (optionally) four more digits.
L0L 0L0. A British or Canadian postal code, which is a pattern of six characters that alternate between characters and digits, like M6S 3H2.
99:00:00 >LL. A mask for entering time information into a Date/Time field. Itâs made up of two digits for the hour and two digits for the minute. The last two characters are always displayed in uppercase (thanks to the > character), and are meant to be AM or PM. (Technically, this mask doesnât prevent the user from flouting the system and typing in two different characters. However, if you enter a time like 12:30 GM, Access complains that it canât convert your entry into the Date/Time data type, as required for the field.)
099.099.099.099. An IP (Internet Protocol) address, which identifies a computer on a network. An IP address is written as four values separated by periods. Each value must have at least one digit, and can have up to three. This patternâs represented in the mask by 099 (one required digit, followed by two optional digits).
Password. A mask that allows ordinary, unlimited text, with one difference. All characters are displayed as asterisks (*), to hide them from prying eyes.
Masks can also have two optional bits of information at the end, separated by semicolons (;).
The second sectionâs a number that tells Access whether or not it should store the literal characters for the mask in the record. (This is the last question that the Input Mask wizard asks.) If you leave this piece out or use the number 1, then Access stores only the characters that someone types in. If you use the number 0, then Access stores the full text with the literals.
The third section supplies the placeholder character. If you leave this section out, then Access uses the familiar underscore.
Hereâs a mask that uses these two extra bits of information:
(000) 000-000;1;#
Here, the second section is 1, and the third section is #. This mask is for phone numbers. It uses the number sign for a placeholder and it includes the following literals: two parentheses, a space, and a dash.
Figure 4-13. To add your own mask, use the record scrolling buttons (at the bottom of this window) to scroll to the end. Or you can use this window to change a mask. For example, the prebuilt telephone mask doesnât require an area code. If thatâs a liberty youâre not willing to take, then replace it with the more restrictive version (000) 000-0000.
Input masks are a great tool, but they apply to only a few specific types of informationâusually fixed-length text that has a single, unchanging pattern. To create a truly bulletproof table, you need to use more sophisticated restrictions, like making sure a number falls in a certain range, checking that a date hasnât yet occurred, or verifying that a text value starts with a certain letter. Validation rules can help you create all these restrictions by drawing on the full power of the SQL language.
Note
Youâll get a more thorough introduction to SQL starting in Chapter 6. Fortunately, you need only a dash of SQL to write a validation rule. The key ingredientâs a validation expression, and youâll see several practical examples of expressions that you can drop straight into your tables.
A validation ruleâs premise is simple. You set up a restriction that tells Access which values to allow in a field and which ones are no good. Whenever someone adds a new record or edits a record, Access makes sure the data lives up to your validation rules. If it doesnât, then Access presents an error message and forces you to edit the offending data and try again.
Each field can have a single validation rule. The following set of steps show you how to set one up. Youâll start out easy, with a validation rule that prevents a numeric field from accepting 0 or any negative number (and in the following sections youâll hone your rule-writing abilities so you can tackle other data types).
Hereâs how to add your validation rule:
In Design view, select the field to which you want to apply the rule.
All data typesâexcept Memo, AutoNumber, and OLE Objectâsupport validation. The validation rule in this example works with any numeric data type (like Number or Currency).
In the Validation Rule field property, type a validation expression (Figure 4-14).
An expressionâs a bit of SQL that performs a check on the data youâve entered. Access performs its validation check when you finish entering a piece of data, and try to navigate to another field or another record. For example, >0 is a validation rule that forces the value in a Number field to be larger than 0. Youâll learn more validation rules in the following sections.
Type some error-message text in the Validation Text field property.
If you enter a value that fails the validation check, then Access rejects the value and displays this error text in a dialog box. If you donât supply any text, then Access shows the validation rule for the field (whatever you entered in step 2), which is more than a little confusing for most mere mortals.
Right-click the tab title, and then choose Datasheet View.
If your table has existing records, Access gives you the option of checking them to make sure they meet the requirements of your validation rule. You decide whether you want to perform this check, or skip it altogether.
Once youâre in Datasheet view, youâre ready to try out your validation rule (Figure 4-15).
Figure 4-15. Here, a validation rule of >0 prevents negative numbers in the Price field. When you enter a negative number, Access pops up a message box with the validation text you defined, as shown here. Once you click OK, you return to your field, which remains in edit mode. You can change the value to a positive number, or press Esc to cancel the record edit or insertion.
Note
Just because your table has validation rules doesnât mean the data inside follows these rules. A discrepancy can occur if you added records before the validation rules came into effect. (You learned about the same potential problem with required fields in Section 4.1.1.) To avoid these headaches, set up your validation rules before you start adding data.
As you can see, itâs easy enough to apply a validation rule to a field. But creating the right validation rule takes more thought. In order to get the result you want, you need to take your first step into the sometimes quirky world of SQL.
Although validationâs limited only by your imagination, Access pros turn to a few basic patterns again and again. The following sections give you some quick and easy starting points for validating different data types.
Note
Access uses your validation rule only if a field contains some content. If you leave it blank, then Access accepts if without any checks. If this isnât the behavior you want, then just set the Required property to Yes to make the field mandatory, as described in Section 4.1.1.
For numbers, the most common techniqueâs to check that the value falls in a certain range. In other words, you want to check that a numberâs less than or greater than another value. Your tools are the comparison signs < and >. Table 4-3 shows some common examples.
Table 4-4. Expressions for Numbers
|
Comparison |
Sample Expression |
Description |
|---|---|---|
|
Less than |
<100 |
The value must be less than 100. |
|
Greater than |
>0 |
The value must be greater than 0. |
|
Not equal to |
<>42 |
The value can be anything except 42. |
|
Less than or equal to |
<=100 |
The value must be less than or equal to 100. |
|
Greater than or equal to |
>=0 |
The value must be greater than or equal to 0. |
|
Equal to |
=42 |
The value must be 42. (Not much point in asking anyone to type it in, is there?) |
|
Between |
Between 0 and 100 |
The value must be 0, 100, or somewhere in between. |
As with numbers, date validation usually involves checking to see if the value falls within a specified range. Here, your challenge is making sure that your dateâs in the right format for an expression. If you use the validation rule >Jan 30, 2007, Access is utterly confused, because it doesnât realize that the text (Jan 30, 2007) is supposed to represent a date. Similarly, if you try >1/30/07, then Access assumes the numbers on the right are part of a division calculation.
To solve this problem, use Access universal date syntax, which looks like this:
#1/30/2007#
A universal date always has the date components in the order month/day/year, and itâs always bracketed by the # symbol on either side. Using this syntax, you can craft a condition like >#1/30/2007#, which states that a given date must be larger than (fall after) the date January 30, 2007. January 31, 2007 fits the bill, but a date in 2006 is out.
The universal date syntax can also include a time component, like this:
#1/30/2007 5:30PM#
Note
When comparing two dates, Access takes the time information into consideration. The date #1/30/2007# doesnât include any time information, so itâs treated as though it occurs on the very first second of the day. As a result, Access considers the date value #1/30/2007 8:00 AM# larger, because it occurs eight hours later.
Once youâve learned the universal date syntax, you can use any of the comparison operators you used with numbers. You can also use these handy functions to get information about the current date and time:
Date() gets the current date (without any time information, so it counts as the first second of the day).
Now() gets the current instant in time, including the date and time information.
Note
A functionâs a built-in code routine that performs some task, like fetching the current date from the computer clock. Youâll learn about many more date functions, which let you perform advanced tasks like finding the day of the week for a date, in Section 7.2.6.
Table 4-4 has some examples.
Table 4-5. Expressions for Dates
|
Comparison |
Sample Expression |
Description |
|---|---|---|
|
Less than |
<#1/30/2007# |
The date occurs before January 30, 2007. |
|
Greater than |
>#1/30/2007 5:30 PM# |
The date occurs after January 30, 2007, or on January 30, 2007, after 5:30 p.m. |
|
Less than or equal to |
<=#1/30/2007# |
The date occurs before January 30, 2007, or on the first second of January 30, 2007. |
|
Greater than or equal to |
>=#1/30/2007# |
The date occurs on or after January 30, 2007. |
|
Greater than the current date |
>Date( ) |
The date occurs today or after. |
|
Less than the current date |
<Date( ) |
The date occurs yesterday or before. |
|
Greater than the current date (and time) |
The date occurs today after the current time, or any day in the future. | |
|
Less than the current date (and time) |
<Now( ) |
The date occurs today before the current time, or any day in the past. |
With text, validation lets you verify that a value starts with, ends with, or contains specific characters. You perform all these tasks with the Like operator, which compares text to a pattern.
This condition forces a field to start with the letter R:
Like "R*"
The asterisk (*) represents zero or more characters. Thus, the complete expression asks Access to check that the value starts with R (or r), followed by a series of zero or more characters.
You can use a similar expression to make sure a piece of text ends with specific characters:
Like "*ed"
This expression allows the values talked, walked, and 34z%($)#ed, but not talking, walkable, or 34z%($)#.
For a slightly less common trick, you can use more than one asterisk. The following expression requires that the letter a and b appear (in that order but not necessarily next to each other) somewhere in a text field:
Like "*a*b*"
Along with the asterisk, the Like operator also supports a few more characters. You can use ? to match a single character, which is handy if you know how long text should be or where a certain letter should appear. Hereâs the validation rule for an eight-character product code that ends in 0ZB:
Like "?????0ZB"
The # character plays a similar role, but it represents a number. Thus, the following validation rule defines a product code that ends in 0ZB and is preceded by five numbers:
Like "#####0ZB"
And finally, you can restrict any character to certain letters or symbols. The trickâs to put the allowed characters inside square brackets.
Suppose your company uses an eight-character product code that always begins with A or E. Hereâs the validation rule you need:
Like "[AE]???????"
Note that the [AE] part represents one character, which can be either A or E. If you wanted to allow A, B, C, D, youâd write [ABCD] instead, or youâd use the handy shortcut [A-D], which means âallow any character from A to D, including A and D.â
Hereâs one more validation expression, which allows a seven-letter word, and doesnât allow numbers or symbols. It works by repeating the [A-Z] code (which allows any letter) seven times.
Like [A-Z][A-Z][A-Z][A-Z][A-Z][A-Z][A-Z]
As you can see, text validation expressions arenât always pretty. Not only can they grow to ridiculous sizes, but there are lots of restrictions they canât apply. You canât, for instance, let the length of the text vary between a minimum and maximum that you set. And you canât distinguish between capitalized and lowercase letters.
Note
You can get around many of these limitations using some of the functions that Access provides. in Section 7.2.5, youâll learn how to use functions that can snip out bits of text, test lengths, check capitalization, and more.
No matter what the data type, you can also combine your conditions in two different ways. Using the And keyword, you can create a validation rule that enforces two requirements. This trickâs handy, because each field can have at most a single validation rule.
To use the And keyword, just write two validation rules and put the word And in between. It doesnât matter which validation ruleâs first. Hereâs a validation rule that forces a date to be before today but later than January 1, 2000:
<Date() And >#1/1/2000#
You can also use the Or keyword to accept a value if it meets either one of two conditions. Hereâs a validation rule that allows numbers greater than 1000 or less thanâ1000:
>1000 Or <-1000
Field validation rules always apply to a single field. However, database designers often need a way to compare the values in different fields. Suppose you have an Orders table that logs purchases from your monogrammed sock store. In your Orders table, you use two date fields: DateOrdered and DateShipped. To keep everything kosher, you need a validation rule that makes sure DateOrdered falls before DateShipped. After all, how can you ship a product out before someone orders it?
Because this validation rule involves two fields, the only way to put it in place is to create a validation rule for the whole table. Table validation rules can use all the SQL tricks youâve learned about so far, and they can pull the values out of any field in the current record.
Hereâs how to create a table validation rule:
In Design view, choose Table Tools | Design â Show/Hide â Property Sheet.
A box with extra settings appears on the right side of the window (Figure 4-16).
Note
You can create only a single validation rule for a table. This limit might sound like a problem, but you can get around it by using the And keyword (Section 4.3.2.4) to yoke together as many conditions as you want. The validation rule may be a little difficult to read, but it still works without a hitch.
Figure 4-16. The Property Sheet shows some information about the entire table, including the sorting (Section 3.2) and filtering settings (Section 3.2.2) youâve applied to the datasheet, and the table validation rule. Here, the validation rule prevents orders from being shipped before theyâre ordered.
In the Property Sheet tab, set the Validation Rule.
A table validation rule can use all the same keywords you learned about earlier. However, table validation rules usually compare two or more fields. The validation rule [DateOrdered] < [DateShipped] ensures that the value for the Date-Ordered field is older than that used for the DateShipped.
When referring to a field in a table validation rule, you need to include square brackets around your field names. That way, Access can tell the difference between fields and functions (like the Date( ) function you learned about in Section 4.1.3).
Set the Validation Text.
This message is the error message thatâs shown if the validation fails. It works the same as the validation text for a field rule.
When you insert a new record, Access checks the field validation rules first. If your data passes the test (and has the right data types), then Access checks the table validation rule.
In a database, minor variations can add up to big trouble. Suppose youâre running International Cinnamon, a multinational cinnamon bun bakery with hundreds of orders a day. In your Orders table, you have entries like this:
Quantity Product 10 Frosted Cinnamon Buns 24 Cinnamon Buns with Icing 16 Buns, Cinnamon (Frosted) 120 FCBs â¦
(Other fields, like the ID column and the information about the client making the order, are left out of this example.)
All the orders shown here amount to the same thing: different quantities of tasty cinnamon and icing confections. But the text in the Product columnâs slightly different. This difference doesnât pose a problem for ordinary human beings (for example, youâll have no trouble filling these orders), but it does create a small disaster if you want to analyze your sales performance later. Since Access has no way to tell that a Frosted Cinnamon Bun and an FCB are the same thing, it treats them differently. If you try to total up the top-selling products or look at long-range cinnamon sales trends, then youâre out of luck.
Note
This example emphasizes a point that youâve seen before. Namely, databases are strict, no-nonsense programs that donât tolerate minor discrepancies. In order for your databases to be useful, you need to make sure you store top-notch information in them.
Lookups are one more tool to help standardize your data. Essentially, a lookup lets you fill a value in a field by choosing from a ready-made list of choices. Used properly, this tool solves the problem in the Orders tableâyou simply need a lookup that includes all the products you sell. That way, instead of typing the product name in by hand, you can choose Frosted Cinnamon Buns from the list. Not only do you save some time, but you also avoid variants like FCBs, thereby ensuring that the orders list is consistent.
Access has two basic types of lookup lists: lists with a set of fixed values that you specify, and lists that are drawn from a linked table. In the next section, youâll learn how to create the first type. Then, in Chapter 5, youâll graduate to the second.
Note
The following data types donât support lookups: Memo, Date/Time, Currency, AutoNumber, Yes/No, OLE Object, Hyperlink, and Attachment.
Simple lookups make sense if you have a simple, short list thatâs unlikely to change. The state prefix in an address is a perfect example. In this case, thereâs a set of just 50 two-letter abbreviations (AL, AK, AZ, and so on).
To try out the process in the following list of steps, you can use the Bachelors table included with the online examples for this chapter (look for the DatingService.accdb database file). Or, you can jump straight to the completed lookup by checking out the DatingServiceLookup.accdb file:
Open the table in Design view.
If youâre using the DatingService.accdb example, then open the Bachelors table.
Find the field where you want to add the lookup.
In the Bachelors table, itâs the State field.
Make sure your field has the correct data type.
Text and Number are the most common data types that youâll use in conjunction with the lookup feature.
Choose Lookup Wizard from the data type list.
This action doesnât actually change your data type. Instead, it tells Access you want to run the Lookup wizard based on the current data type. When you select this option, the first step of the Lookup wizard appears (Figure 4-17).
Choose âI will type in the values that I needâ.
Section 5.2.5 describes your other choice: drawing the lookup list from another table.
Click Next.
The second step of the wizard gives you the chance to supply the list of values that should be used, one per row (Figure 4-18). In this case, itâs a list of abbreviations for the 50 U.S. states.
You may notice that you can supply multiple columns of information. For now, stick to one column. Youâll learn why you may use more in Section 5.2.5.
Click Next.
The final step of the Lookup wizard appears.
Choose whether or not you want the lookup column to store multiple values.
If you allow multiple values, then the lookup list displays a checkbox next to each item. You can select several values for a single record by checking more than one item.
In the State field, it doesnât make sense to allow multiple valuesâafter all, a person can physically inhabit only one state (discounting the effects of quantum teleportation). However, you can probably think of examples where multiple selection does make sense. For example, in the Products table used by International Cinnamon, a multiple-value lookup would let you create an order for more than one product. (Youâll learn more about multiple value selections and table relationships in Chapter 5.)
Click Finish.
Switch to Datasheet view (right-click the tab title, and then choose Datasheet View), and then save the table changes. Figure 4-19 shows the lookup in action.
When you create a lookup that uses fixed values, the lookup list provides a list of suggestions. You can choose to ignore the lookup list and type in a completely different value (like a state prefix of ZI), even if it isnât on the list. This design lets you use the lookup list as a timesaving convenience without limiting your flexibility.
In many cases, you donât want this behavior. In the Bachelors table, you probably want to prevent people from entering something different in the State field. In this case, you want the lookup to be an error-checking and validation tool that actually stops entries that donât belong.
Fortunately, even though this optionâs mysteriously absent in the Lookup wizard, itâs easy enough to add after the fact. Hereâs what you need to do:
In Design view, go to the field that has the lookup.
In the Field Properties section, click the Lookup tab.
The Lookup tab provides options for fine-tuning your lookup, most of which you can configure more easily in the Lookup wizard. In the Row Source box, for example, you can edit the list of values you supplied. (Each valueâs on the same line, in quotation marks, separated from the next value with a semicolon.)
Set the Limit to List property to Yes.
This action prevents you from entering values that arenât in the list.
Optionally, set Value List Edits to Yes.
This action lets people modify the list of values at any time. This way, if somethingâs missing from the lookup list, you can add it on the fly (Figure 4-20).
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