Q253595

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Q253595: HOWTO: Build Indexes with DateTime Fields and Other Data Types

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Article: Q253595
Product(s): Microsoft FoxPro
Version(s): MACINTOSH:3.0b; WINDOWS:3.0,3.0b,5.0,5.0a,6.0
Operating System(s): 
Keyword(s): kbDatabase kbvfp300b kbvfp500 kbvfp500a kbvfp600 kbXBase KbDBFDBC kbGrpDSFox kbDSupport
Last Modified: 29-FEB-2000

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, versions 3.0, 3.0b, 5.0, 5.0a, 6.0 
- Microsoft Visual FoxPro for Macintosh, version 3.0b 
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SUMMARY
=======

Sometimes there is a need to create indexes using DateTime fields along with
other fields in your table. To index on multiple fields of different data types,
it is best to convert each non-character field value into a character value
using the appropriate conversion function. The function TTOC() provides an
indexable character value from a DATETIME value when used with the second
parameter value of 1. This article provides examples of how to use this function
to create various indexes using a DateTime field.

MORE INFORMATION
================

By default, indexes are created in ascending order. If you include the
DESCENDING keyword, the index is in descending order. Sometimes you might need
to build an index on a field in ascending sequence and a second field in
descending sequence. To achieve this result, you must calculate a value for the
second field that decreases in value as the actual value increases. For a
DateTime field, if you subtract the numeric value of the character value
returned by the TTOC() function from the number 99999999999999, you obtain a
value that decreases as the actual value increases. By converting this
calculated value to a character value, you can combine this with another
character value to achieve the desired results.

The TTOC() function, with the second parameter having the value 1, returns a
character value from a DATETIME value in the format YYYYMMDDhhmmss. If you enter
the following statement in the Command window, you see the current system date
and time presented in this format:

  ? TTOC( DATETIME(), 1 )

To see examples of various indexes with an integer field and a DateTime field,
create a program called EXAMPLE.prg and paste the following code into the file:

  #DEFINE ONEDAY 86400
  LOCAL lcTag, x

  CLEAR

  CREATE TABLE Example (iRecID I, tDateTime T, cName C(10))

  INSERT INTO Example (iRecID, tDateTime, cName) ;
     VALUES (1, DATETIME(), "Record 1")
  INSERT INTO Example (iRecID, tDateTime, cName) ;
     VALUES (1, DATETIME() + ONEDAY, "Record 2")

  INSERT INTO Example (iRecID, tDateTime, cName) ;
     VALUES (2, DATETIME(), "Record 3")
  INSERT INTO Example (iRecID, tDateTime, cName) ;
     VALUES (2, DATETIME() - ONEDAY, "Record 4")

  INSERT INTO Example (iRecID, tDateTime, cName) ;
     VALUES (3, DATETIME(), "Record 5")
  INSERT INTO Example (iRecID, tDateTime, cName) ;
     VALUES (3, DATETIME() + ONEDAY, "Record 6")

  INSERT INTO Example (iRecID, tDateTime, cName) ;
     VALUES (4, DATETIME(), "Record 7")
  INSERT INTO Example (iRecID, tDateTime, cName) ;
     VALUES (4, DATETIME() - ONEDAY, "Record 8")
     
  INDEX ON iRecID    TAG T1
  INDEX ON tDateTime TAG T2
  INDEX ON STR(iRecID) + TTOC(tDateTime, 1) TAG T3
  INDEX ON STR(iRecID) + TTOC(tDateTime, 1) TAG T4 DESCENDING
  INDEX ON STR(iRecID) + ;
  	STR(99999999999999 - VAL(TTOC(tDateTime, 1)), 14, 0) TAG T5
  	
  SET ALTERNATE TO Example.txt
  SET TALK ON
  SET ALTERNATE ON

  FOR x = 1 TO 5
     lcTag = "T" + LTRIM(STR(x))
     SET ORDER TO TAG (lcTag)
     ? "Index Tag " + TAG() + ;
       ", index expression: " + KEY() + ;
       ", sequence: " + IIF(DESCENDING(), "Descending", "Ascending")
     LIST
  ENDFOR

  SET ALTERNATE OFF
  SET TALK OFF
  SET ALTERNATE TO
  USE

  MODIFY FILE Example.txt

The above program creates a table consisting of an Integer field, a DateTime
field, and a Character field. It then adds five records to the table. The
program then creates five index tags for the following situations:

- Integer field in ascending sequence
- DateTime field in ascending sequence
- Integer and DateTime fields in ascending sequence
- Integer and DateTime fields in descending sequence
- Integer field in ascending sequence and the DateTime field in descending
  sequence

After creating the index tags, the program creates a text file and lists the five
records, each time using a different index tag. Once this is completed, the
program displays the results so that you can see how the various indexing
techniques affect the sequence of the records in the table.

REFERENCES
==========

For additional information on indexing, click the article number below to view
the article in the Microsoft Knowledge Base:

  Q97066 How to Index a Combined Date and Character Field

Additional query words:

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Keywords          : kbDatabase kbvfp300b kbvfp500 kbvfp500a kbvfp600 kbXBase KbDBFDBC kbGrpDSFox kbDSupport kbCodeSnippet 
Technology        : kbHWMAC kbOSMAC kbVFPsearch kbAudDeveloper kbVFP300bMac kbVFP300 kbVFP300b kbVFP500 kbVFP600 kbVFP500a
Version           : MACINTOSH:3.0b; WINDOWS:3.0,3.0b,5.0,5.0a,6.0
Issue type        : kbhowto

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