And operator to display orders shipped no earlier than 5-Jan and no later than Jan And operator to display orders required between today's date and three months from today's date. The expressions in the following table work with fields that have potentially missing information — those that might contain a null value or a zero-length string.
A null value represents the absence of information; it does not represent a zero or any value at all. Access supports this idea of missing information because the concept is vital to the integrity of a database. In the real world, information is often missing, even if only temporarily for example, the as-yet undetermined price for a new product. Therefore, a database that models a real world entity, such as a business, must be able to record information as missing.
You can use the IsNull function to determine if a field or control contains a null value, and you can use the Nz function to convert a null value to zero. Displays orders for customers who don't have a fax machine, indicated by a zero-length string value in the Fax field instead of a null missing value. The Like operator provides a great deal of flexibility when you are trying to match rows that follow a pattern, because you can use Like with wildcard characters and define patterns for Access to match.
For more information, see the article Like Operator. Finds all records in the ShipName field that include "Maison" in the first part of the value and a five-letter string in which the first four letters are "Dewe" and the last letter is unknown. You use an SQL or domain aggregate function when you need to sum, count, or average values selectively.
For example, you might want to count only those values that fall within a certain range, or that evaluate to Yes. At other times, you might need to look up a value from another table so that you can display it. The sample expressions in the following table use the domain aggregate functions to perform a calculation on a set of values, and use the result as the query criteria.
Uses the DStDev and DAvg functions to display all orders for which the freight cost rose above the mean plus the standard deviation for freight cost. Uses the DAvg function to display products ordered in quantities above the average order quantity.
You use a subquery, also called a nested query, to calculate a value for use as a criterion. The sample expressions in the following table match rows based on the results returned by a subquery. Salary of every sales representative whose salary is higher than that of all employees with "Manager" or "Vice President" in their titles. You use an update query to modify the data in one or more existing fields in a database.
For example, you can replace values or delete them entirely. This table demonstrates some ways to use expressions in update queries. You use these expressions in the Update To row in the query design grid for the field that you want to update.
For more information about creating update queries, see the article Create and run an update query. Every query that you create in query Design view can also be expressed by using SQL. The following table shows sample SQL statements that employ an expression. Calculates the average extended price for orders for which the value in the ExtendedPrice field is more than , and displays it in a field named Average Extended Price.
In a field named CountOfProductID, displays the total number of products for categories with more than 10 products. The two most common ways to use expressions in tables are to assign a default value and to create a validation rule. When you design a database, you might want to assign a default value to a field or control.
Access then supplies the default value when a new record containing the field is created or when an object that contains the control is created. The expressions in the following table represent the sample default values for a field or control.
If a control is bound to a field in a table, and the field has a default value, the default value of the control takes precedence. New York, N. Note that you must enclose the value in quotation marks if it includes punctuation. A zero-length string to indicate that, by default, this field should be empty instead of containing a null value. You can create a validation rule for a field or control by using an expression. Access then enforces the rule when data is entered into the field or control.
To create a validation rule, you modify the ValidationRule property of the field or control. You should also consider setting the ValidationText property, which holds the text that Access displays when the validation rule is violated. If you don't set the ValidationText property, Access displays a default error message. The examples in the following table demonstrate the validation rule expressions for the ValidationRule property and the associated text for the ValidationText property.
For more information about validating data, see the article Create a validation rule to validate data in a field. In some cases, you might want to carry out an action or series of actions in a macro only if a particular condition is true.
For example, suppose you want an action to run only when the value of the Counter text box is You use an expression to define the condition in the Condition column of the macro:. As with the ValidationRule property, the Condition column expression is a conditional expression. It must resolve to either a True or False value. The action takes place only when the condition is true. Tip To force Access to temporarily ignore an action, type False as a condition.
This can be helpful when you are trying to find problems in a macro. The value of the ShippedDate field on the form from which the macro is run is no earlier than 2-Feb and no later than 2-Mar The FirstName value on the form from which the macro is run is null has no value. This expression is equivalent to [FirstName] Is Null. The value in the CountryRegion field on the form from which the macro is run is UK, and the value of the TotalOrds field on the SalesTotals form is greater than The value in the CountryRegion field on the form from which the macro is run is either France, Italy, or Spain, and the postal code is not 5 characters long.
You click OK in a dialog box that the MsgBox function displays. If you click Cancel in the dialog box, Access ignores the action. Use the Expression Builder. Introduction to expressions. Build an expression. Guide to expression syntax. If you want to combine those first and last names and then display them in a single field, you can create a calculated field in the table or in a query:. You can also use expressions in Access to provide a default value for a field in a table or for a control.
For example, to set the default value for a date field to the current date, in the Default Value property box for that field, you type:. In addition, you can use an expression to set a validation rule. For example, you can use a validation rule in a control or table field to require that the date that is entered is greater than or equal to the current date.
In that case, you set the value in the Validation Rule property box to:. Finally, you can use an expression to set criteria for a query.
For example, suppose that you want to see product sales for the orders that were shipped within a certain time frame. You can enter criteria to define a date range, and Access returns only the rows that match the criteria.
For example, your expression might look like this:. When you add criteria to the query and then run the query, it returns only those values that match the specified dates. Calculates the difference between the date values in two text box controls called RequiredDate and ShippedDate on a report. Sets a validation rule for a numeric field in a table — users must enter values greater than zero. An expression consists of a number of possible components that you can use, alone or in combination, to produce a result.
These components include:. Identifiers The names of table fields or controls on forms or reports, or the properties of those fields or controls. Constants Values that do not change — such as strings of text, or numbers that are not calculated by an expression. Values Strings, such as "Enter a number between 1 and An identifier is the name of a field, property, or control.
You use an identifier in an expression to refer to the value that is associated with a field, property, or control.
This expression subtracts the value of the ShippedDate field or control from the value of the RequiredDate field or control. In this expression, both RequiredDate and ShippedDate serve as identifiers.
Functions are built-in procedures that you can use in your expressions. You use functions for a wide variety of operations, such as calculating values, manipulating text and dates, and summarizing data.
For example, one commonly used function is DATE, which returns the current date. You can use the DATE function in a variety of ways, such as in an expression that sets the default value for a field in a table. The applications included in the Expression Studio pack by Microsoft , stand out because they are professionally focused tools that are modular and complete.
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