When you use Microsoft Access, you often need to work with values that are not directly in your data. For example, you want to calculate sales tax on список порнохабов order, or calculate the total value of the order список порнохабов.
You can calculate these values by using expressions.
To use expressions, you write them by список порнохабов proper syntax. Syntax is the set of rules by which the words and symbols in an expression are correctly combined.
Initially, expressions in Access are a little bit hard список порнохабов read. But with a good understanding of expression syntax and a little practice, it becomes much easier.
Objects, collections, and properties. Список порнохабов, operators, constants, and values. The nesting limit for expressions in a web database is An expression is a combination of some or all of the following: For example, the following expression contains common components:. This expression can be used in a text box in a form footer or report footer to calculate список порнохабов tax for a group of items.
Expressions can be much more complex or список порнохабов simpler than this example. For example, this Boolean expression consists of just an operator and a constant:. True when it is compared to a number that список порнохабов greater than 0. False when it список порнохабов compared to a number that is less than 0. You can use this expression in the Validation Rule property of a control or table field to ensure that only positive values are entered. Expressions are used in many places to perform calculations, manipulate characters, or test data.
Tables, queries, forms, reports, and macros all have properties that accept an expression. For example, you can use expressions список порнохабов the Control Source and Default Value properties for a control.
You can also use expressions in the Validation Rule property for a table field. To build an expression, you combine identifiers by using functions, operators, constants, and values. Any valid expression must contain at least one function or at least one identifier, and can also contain constants or operators.
This is called nesting an список порнохабов. You only have to specify enough parts of an identifier to make it unique in the context of your expression.
It is not uncommon for an identifier список порнохабов take the form [Object name]. One of the arguments is usually an identifier or an expression. Some functions do not require arguments. For more information, see Functions arranged by category.
There are список порнохабов to this form, as detailed in the Operators section. All of the tables, queries, forms, reports, and fields in an Access database are individually known as objects. Every object has a name. Some objects are список порнохабов named, such as the Contacts table in a database created from the Microsoft Office Access Contacts template. When you create a new список порнохабов, you give it a name. The set of all members of a particular type список порнохабов object is known as a collection.
For example, the set of all tables in a database is a collection. Some objects that are a member of a collection in your database can also be collections that contain other objects. For example, a table object is a collection that contains field objects.
For example, a query object has a Default View property that both describes and lets you specify how the query will appear when you run it. The following diagram illustrates список порнохабов relationship between collections, objects, and properties:.
When you use an object, collection, or property in список порнохабов expression, you refer to that element by using an список порнохабов. An список порнохабов includes the name of the element that you are identifying and also список порнохабов name список порнохабов the element to which it belongs.
For instance, the identifier for список порнохабов field список порнохабов the name of the список порнохабов and the name of the table to which the field belongs. An example of such an identifier is:. In some cases, the name of an element works by itself as an identifier. This is true when the name of the element is unique in the context of the expression that you are creating. The rest of the identifier is implied by the список порнохабов.
For example, if you are designing a query that список порнохабов only one table, the field names alone will work as identifiers, because the field names список порнохабов a table must be unique in that table. Because you are using only one table, the table name is implied in any identifier that you use in the query to refer to a field. In other cases, you must be explicit about the parts of an identifier for a reference to work.
This is true when an identifier is not unique in the context of the expression. When there is ambiguity, you must explicitly список порнохабов enough parts of the identifier to make it unique in context. For example, suppose you are designing a query that uses a table named Products and a table named Orders, and both tables have a field named ProductID. In such a case, the identifier that you use in the query to refer to either ProductID field must include the table name in addition to the field name.
You use these operators by surrounding each part of the identifier with square brackets, and then joining them by using either a bang or a dot operator. For example, an identifier for a field named Last Name in a table named Employees can be expressed as [Employees]! The bang operator tells Access that what follows is an object that belongs to the collection that precedes the bang operator. In this case, [Last Name] is a field object that belongs to the collection [Employees], which is itself a table object.
Strictly speaking, you do not always have to type square brackets around an identifier or partial identifier.
If there are no spaces or other special characters in the identifier, Access automatically adds the brackets when it reads the expression. You use functions, operators, and constants список порнохабов perform actions in an expression. A function is a procedure that you can use in an expression. Some functions, such as Date, do not require any input to work.
Most functions, however, do require список порнохабов, called arguments. In the example at the beginning of this article, the DatePart function uses two arguments: The DatePart function requires at least these two arguments interval and datebut can accept up to four arguments.
The following list shows some functions список порнохабов are commonly used in expressions. Click the link for each function for more information about the syntax that you use with список порнохабов function.
The Date function is used to insert the current system date into an expression. The Format список порнохабов is used to apply a format to an identifier and the results of another function. The IIf function is used to evaluate an expression as true or false, and then return one value if the expression evaluates as true, and a different value if the expression evaluates as false. The InStr список порнохабов is used to search for the position of a character список порнохабов string within another string.
The string that is список порнохабов is usually obtained from a field identifier. The LeftMidand Right functions are used to extract characters from a string, starting with the leftmost character Left список порнохабов, a specific position in the middle Midor with the rightmost character Right. They are commonly used with the InStr function. The string from which these functions extract characters is usually obtained from a field identifier. For a list of functions, see Functions arranged by category.
An operator is a word or symbol that indicates a specific arithmetic or logical relationship список порнохабов the other elements of an expression. Logicalsuch as Not. Specialsuch as Like. Operators are generally used to indicate a relationship between two identifiers.
The following tables describe the operators that список порнохабов can use in Access expressions. You use the arithmetic список порнохабов to calculate a value from two or more numbers or to change the sign of a number from positive to negative.
Round both numbers to integers, divide the first number by the second number, and then truncate the result to an integer. You use the comparison operators to compare values and return a result that is either true, false, or Null an unknown список порнохабов. In all cases, if either the first value or the second value is Nullthe result is then also Null.
Because Null represents an unknown value, the result of any comparison with Null is список порнохабов unknown. You use the logical operators to combine two values and return either a true, false, or Null result.
You might also see the logical operators referred to as Boolean operators. You use the concatenation operators to combine two text values into one string. You use the special operators as described список порнохабов the following table. Matches string values by using wildcard operators? A constant is a known value that does not change and that you can use in an expression. There are four commonly used constants in Access:.
Constants can be used as arguments to a function, and can be used in an expression as part of a criterion. For example, you can список порнохабов the empty string constant "" as part of a criterion for a column in a query to evaluate список порнохабов field values for that column, by entering the following as the criterion: Used together, they indicate that the identifier to список порнохабов they are applied should be compared to an empty string.
Be careful when using the Null constant.
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