ACC2000: How to Compare a Field to a Field in a Prior Record (208953)

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

Note that you receive the following results from the query or the stored procedure:

   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