Microsoft KB Archive/208953

= ACC2000: How to Compare a Field to a Field in a Prior Record =

Article ID: 208953

Article Last Modified on 6/29/2004

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APPLIES TO


 * Microsoft Access 2000 Standard Edition

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This article was previously published under Q208953



Advanced: Requires expert coding, interoperability, and multiuser skills.

This article applies to a Microsoft Access database (.mdb) and to a Microsoft Access project (.adp).



SUMMARY
This article shows you how to use a subquery to compare the values in a record with the values in another record. In order to use the method described in this article, the table that you are using must have one or more fields that can be compared by using the Greater Than (>) or Less Than (<) operator.



MORE INFORMATION
The following example demonstrates how to calculate miles per gallon by subtracting a recorded mileage from the current mileage, and then dividing by the number of gallons of gasoline recorded in the current record. The example is divided into two sections: one for an Access database, and the other for an Access project.

In a Microsoft Access database (MDB)
  Create the following new table, and then save it as MileageRecord. Do not create a primary key for the table:   Table: MileageRecord Field Name: Date Data Type: Date/Time Field Name: Mileage Data Type: Number Field Size: Single Field Name: Gallons Date Type: Number Field Size: Double   View the table in Datasheet view, and enter the following records in the table:   Date        Mileage   Gallons -  7/08/1999   12340     14.8   7/13/1999   12700     12.6   7/18/1999   13090     13.7   7/25/1999   13425     11.9                     Create a new, blank query based on the MileageRecord table. Add the Date and Mileage fields to the query grid. Click the Properties button on the toolbar to view the property sheet, and then select the title bar of the MileageRecord table. Set the Alias property of the table to Mile1.  Enter the following expression in the third column in the query grid.

NOTE: In the following sample expression, an underscore (_) at the end of a line is used as a line-continuation character. Remove the underscore from the end of the line when re-creating this expression. <pre class="fixed_text">  PrevMileage: (Select Max(Mileage) from MileageRecord _   Where Mileage < Mile1.[Mileage]) This expression is a subquery that finds the highest mileage in the MileageRecord table that is less than the mileage of the current record. </li>  Enter the following expression in the fourth column in the query grid: <pre class="fixed_text">  Elapsed: [Mileage] - [PrevMileage] This field calculates the difference between the current and previous mileage entries. </li>  Enter the following expression in the fifth column in the query grid: <pre class="fixed_text">  MPG: ([Mileage] - [PrevMileage]) / Gallons This field calculates the miles per gallon. </li> Run the query.</li></ol>

In a Microsoft Access project (ADP)
<ol>  Create the following new table, and then save it as MileageRecord: <pre class="fixed_text">  Table: MileageRecord Column Name: ID   Datatype: uniqueidentifier Allow Nulls: no   Default Value: newid Column Name: Date Datatype: datetime Column Name: Mileage Datatype: decimal Length: 9 Precision: 18 Scale: 2 Column Name: Gallons Datatype: decimal Length: 9 Precision: 18 Scale: 2 </li>  View the table in Datasheet view, and enter the following records in the table: <pre class="fixed_text">  Date        Mileage   Gallons -  7/08/1999   12340     14.8   7/13/1999   12700     12.6   7/18/1999   13090     13.7   7/25/1999   13425     11.9                    </li>  Create the following stored procedure: CREATE PROCEDURE "Calculate_Mileage_Proc" AS SELECT MileageRecord.Date, MileageRecord.Mileage, (SELECT MAX(mileage)    FROM mileagerecord     WHERE mileagerecord.mileage < mileagerecord1.mileage) AS PrevMileage, MileageRecord.Mileage - (SELECT MAX(mileage)   FROM mileagerecord    WHERE mileagerecord.mileage < mileagerecord1.mileage) AS Elapsed, (MileageRecord.Mileage - (SELECT MAX(mileage) FROM mileagerecord WHERE mileagerecord.mileage < mileagerecord1.mileage)) / MileageRecord.Gallons AS MPG

FROM MileageRecord

INNER JOIN MileageRecord MileageRecord1 ON MileageRecord.id = MileageRecord1.id                    </li> Save and run the stored procedure.</li></ol>

Results
Note that you receive the following results from the query or the stored procedure: <pre class="fixed_text">  Date      Mileage   PrevMileage   Elapsed   MPG 7/08/99  12340   7/13/99   12700     12340         360       28.5714285714286   7/18/99   13090     12700         390       28.4671532846715   7/25/99   13425     13090         335       28.1512605042017

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