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

• Microsoft Access 2000

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)

1. 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 

2. 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 

3. Create a new, blank query based on the MileageRecord table. Add the Date and Mileage fields to the query grid.



4. 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.



5. 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.

   
      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.



6. Enter the following expression in the fourth column in the query grid:

   
      Elapsed: [Mileage] - [PrevMileage] 

   This field calculates the difference between the current and previous mileage entries.



7. Enter the following expression in the fifth column in the query grid:

   
      MPG: ([Mileage] - [PrevMileage]) / Gallons 

   This field calculates the miles per gallon.



8. Run the query.

In a Microsoft Access project (ADP)

1. Create the following new table, and then save it as MileageRecord:




   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 

2. 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 

3. 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 

4. Save and run the stored procedure.

Results

   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 

REFERENCES

For more information about subqueries, click Microsoft Access Help on the Help menu, type "SQL subqueries" in the Office Assistant or the Answer Wizard, and then click Search to view the topic.

Additional query words: differential calculus

Keywords          : kbdta QrySqlvw 
Version           : WINDOWS:2000
Platform          : WINDOWS 
Issue type        : kbhowto 

Last Reviewed: May 26, 1999