Validation text lets you provide a message to help users who input data that is not valid. When data is entered, Access checks to see whether the input breaks a validation rule — if so, the input is not accepted, and Access displays a message. Data types Every table field has a data type that restricts what users can enter. Field properties Some field properties restrict data input.
For example, the Field Size property of a field restricts input by limiting the amount of data. You can also use the Validation Rule property to require specific values, and the Validation Text property to alert your users to any mistakes. Entering text such as "Enter values between and 1," or "Enter an ending date on or after the start date" in the Validation Text property tells users when they have made a mistake and how to fix the error.
Input masks You can use an input mask to validate data by forcing users to enter values in a specific way. For example, an input mask can force users to enter dates in a European format, such as You can use these methods of validating data alone or in combination with each other.
Data types are not optional, and provide the most basic type of data validation. For more information about data types, field sizes, and input masks, see the article Introduction to data types and field properties.
Field validation rules Use a field validation rule to check the value that you enter in a field when you leave the field. Your rule now requires users to enter dates on or after January 1, If you enter a date earlier than and then try to place the focus on another field, Access prevents you from leaving the current field until you fix the problem. Record validation rules Use a record validation rule to control when you can save a record a row in a table.
Unlike a field validation rule, a record validation rule refers to other fields in the same table. You create record validation rules when you need to check the values in one field against the values in another. For example, suppose your business requires you to ship products within 30 days and, if you don't ship within that time, you must refund part of the purchase price to your customer. If the syntax for validation rules looks cryptic, see the section What you can put in a validation rule for an explanation of the syntax and some example validation rules.
You can define validation rules for table fields and for controls on forms. When you define rules for tables, those rules apply when you import data. To add validation rules to a table, you open the table and use commands on the Fields tab of the ribbon. To add validation rules to a form, you open the form in Layout view and add rules to the properties of individual controls. The steps in the section Add a validation rule to a table explain how to add validation rules to table fields.
The steps in the section Add a validation rule to a control on a form , later in this article, explain how to add rules to the properties in individual controls. Your validation rules can contain expressions — functions that return a single value.
You can use an expression to perform calculations, manipulate characters, or test data. A validation rule expression tests data. Expressions can also perform mathematical operations. The expression [OrderDate] - [ShipDate] calculates the number of days that elapsed between the time an order was placed and the time it shipped.
For example, detailed analytics will require, at the very least, knowledge of the number of NULL values in a particular data set. Although this will give us the same results without the message, turning off this option can affect other operations. For example, modifying data on indexed views or on tables with indexed computed columns will fail if the option is turned off. So tread carefully if you consider this route. Also be aware that when you start grouping and aggregating data, you might not get exactly the results you expect when that data includes NULL values.
If we were to dig into the averages and the individual column counts for each group, we would discover that the NULL values are not included, but the result set as a whole includes all the data.
Boolean expressions that involve NULL values can be especially tricky because they introduce what is called three-valued logic. The main thing to keep in mind with Boolean expressions is that your result set will include only those rows in which the expression evaluates to TRUE. Notice that the query engine excludes the row for product ID In this case, only one of the two Boolean conditions needs to evaluate to TRUE , resulting in many more returned rows. Not surprising, the only rows to be included are those in which both Boolean conditions evaluate to TRUE.
You just need to be aware of what can happen with your data when NULL values are involved. Data can be eliminated that you might expect to be included, leading to unintended consequences in subsequent analytics and decision-making. Now suppose when want to filter our data set even further by returning all rows with a Color value other than black :.
Once again, the bottom line is that you must be prepared to handle NULL values if they could in any way impact your results. Another area where we might get results other than what we expect is when we join data based on nullable columns. Because NULL value are considered non-values and because the database engine cannot match non-values, NULL values will show up in the join column only for outer joins. We can also try our statement with a full outer join:.
Like any place where NULL shows up, it comes down to understanding the data we could potentially be dealing with and what that means to the bigger picture. When we join data, a NULL in one column can cause us to miss important data in other columns, all because we did not think through the possible outcomes. We cannot compare or add or multiply NULL values because we cannot compare or add or multiply missing values. Because the column contains multiple NULL values, the database engine returns the following error:.
After all, how can missing values be considered duplicates if there is no way to compare them? Because of this limitation, we have to be particularly careful when we create our tables and indexes prior to populating them. We could be faced with similar error messages if we try to insert a row with a duplicate NULL value. The index is now created without a hitch. We can even insert additional duplicate NULL values:. Creating a filtered index is easy enough to do and is a handy way to sidestep the limitations of a unique index.
A result set that is not in the order you want might not seem such a big deal, but it can still be annoying when you sort your results and end up with a bunch of NULL values at the beginning.
We can instead sort the data in descending order, but that would put the colors themselves in reverse order. From there, the normal sorting occurs, as shown in the following results:. Fortnightly newsletters help sharpen your skills and keep you ahead, with articles, ebooks and opinion to keep you informed. Robert is a freelance technology writer based in the Pacific Northwest.
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Linked Related Hot Network Questions. Field validation rules. All macro expressions. The tables in this section provide examples of expressions that calculate a value in a control located on a form or report. To create a calculated control, you enter an expression in the ControlSource property of the control, instead of in a table field or query.
Note You can also use expressions in a form or report when you Highlight data with conditional formatting. Displays the values that reside in table fields called FirstName and LastName. Uses the Left function to display the first character of the value of a field or control called ProductName. Uses the Right function to display the last 2 characters of the value in a field or control called AssetCode.
Uses the Trim function to display the value of the Address control, removing any leading or trailing spaces.
Uses the IIf function to display the values of the City and PostalCode controls if the value in the Region control is null; otherwise, it displays the values of the City, Region, and PostalCode controls, separated by spaces. Null propagation means that if any component of an expression is null, the entire expression is also null. You use the Page and the Pages properties to display or print page numbers in forms or reports. The Page and Pages properties are available only during printing or print preview, so they do not appear on the property sheet for the form or report.
Typically, you use these properties by placing a text box in the header or footer section of the form or report, and then using an expression, such as the ones shown in the following table.
For more information about using headers and footers in forms and reports, see the article Insert page numbers into a form or report. You can use expressions to add, subtract, multiply, and divide the values in two or more fields or controls. You can also use expressions to perform arithmetic operations on dates. The product of the value of the Price field or control and 1. Sometimes, you need a value that exists somewhere else, such as in a field or control on another form or report.
You can use an expression to return the value from another field or control. The following table lists examples of expressions that you can use in calculated controls on forms. Column 2. The value of the third column in ProductID, a multiple-column list box on the subform named Orders Subform on the Orders form. Note that 0 refers to the first column, 1 refers to the second column, and so on.
The product of the value of the Price control on the subform named Orders Subform on the Orders form and 1. The expressions in the following table show some ways to use calculated controls on reports. The expressions reference the Report Property. You can use a type of function called an aggregate function to calculate values for one or more fields or controls.
For example, you can calculate a group total for the group footer in a report, or an order subtotal for line items on a form. You can also count the number of items in one or more fields or calculate an average value.
The expressions in the following table show some of the ways to use functions such as Avg, Count, and Sum. Uses the Avg function to display the average of the values of a table field or control named "Freight. Uses the Sum function to display the sum of the product of the values of the Quantity and the Price controls. Displays the percentage of sales, determined by dividing the value of the Sales control by the sum of all the values of the Sales control.
For more information about using aggregate functions and totaling the values in field and columns, see the articles Sum data by using a query , Count data by using a query , Display column totals in a datasheet using a Totals row , and Display column totals in a datasheet.
You use a type of function called an SQL or domain aggregate function when you need to sum or count values selectively.
A "domain" consists of one or more fields in one or more tables, or one or more controls on one or more forms or reports. For example, you can match the values in a table field with the values in a control on a form. Tracking dates and times is a fundamental database activity. For example, you can calculate how many days have elapsed since the invoice date to age your accounts receivable. You can format dates and times in numerous ways, as shown in the following table. Uses the Date function to display the current date in the form of mm-dd-yy , where mm is the month 1 through 12 , dd is the day 1 through 31 , and yy is the last two digits of the year through Uses the Format function to display the week number of the year for the current date, where ww represents weeks 1 through Uses the DateAdd function to display a date that is 10 days before the value of the PromisedDate control.
Uses the DateDiff function to display the number of days' difference between the values of the OrderDate and ShippedDate controls. Uses arithmetic operations on dates to calculate the date 30 days after the date in the InvoiceDate field or control.
The example expressions in the following table use the IIf function to return one of two possible values. You pass the IIf function three arguments: The first argument is an expression that must return a True or False value. The second argument is the value to return if the expression is true, and the third argument is the value to return if the expression is false. Uses the IIf and IsNull functions to display the values of the City and PostalCode controls if the value in the Region control is null; otherwise, it displays the values of the City, Region, and PostalCode fields or controls.
Uses the IIf and IsNull functions to display the message "Check for a missing date" if the result of subtracting ShippedDate from RequiredDate is null; otherwise, it displays the interval between the date values of the RequiredDate and ShippedDate controls. This section contains examples of expressions that you can use to create a calculated field in a query or to supply criteria to a query. A calculated field is a column in a query that results from an expression. For example, you can calculate a value, combine text values such as first and last names, or format a portion of a date.
You use criteria in a query to limit the records that you work with. For example, you can use the Between operator to supply a starting and ending date and limit the results of your query to orders that were shipped between those dates. Creates a field called Address2 that displays the values in the City, Region, and PostalCode fields, separated by spaces.
ProductInitial: Left [ProductName], 1. Creates a field called ProductInitial, and then uses the Left function to display, in the ProductInitial field, the first character of the value in the ProductName field. Creates a field called TypeCode, and then uses the Right function to display the last two characters of the values in the AssetCode field.
Creates a field called AreaCode, and then uses the Mid function to display the three characters starting with the second character of the value in the Phone field. Names the calculated field ExtendedPrice, and uses the CCur function to calculate the line item totals with an applied discount. You can also perform arithmetic operations on dates.
Creates a field called PrimeFreight, and then displays freight charges plus 10 percent in the field.
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