Microsoft KB Archive/106708

= How to generate unique key field values in FoxPro and in Visual FoxPro =

Article ID: 106708

Article Last Modified on 2/12/2007

-

APPLIES TO


 * Microsoft Visual FoxPro 3.0 Standard Edition
 * Microsoft FoxPro 2.0
 * Microsoft FoxPro 2.6a Professional Edition for Macintosh
 * Microsoft Visual FoxPro 7.0 Professional Edition
 * Microsoft Visual FoxPro 8.0 Professional Edition
 * Microsoft Visual FoxPro 9.0 Professional Edition

-



This article was previously published under Q106708





SUMMARY
You can use the following methods to generate a unique string for a key field value:
 * If you are using Microsoft Visual FoxPro 8.0 and later versions, you can create an autoincrement field type.
 * You can create a unique value by using Microsoft Windows API functions.
 * If you are using earlier versions of FoxPro and of Visual FoxPro, another method is described later.

For additional information, see the "References" section.



MORE INFORMATION
The following code is adapted from the "FoxPro 2.5 Advanced Developer's Handbook" by Adams and Powell, Chapter 18, page 644.

The following function accepts a numeric parameter to specify the length of the key desired. If no parameter is passed, the function will generate a unique key that is 8 characters long. The maximum key length that can be generated is 18 characters. FUNCTION ukey PARAMETER lenkey IF PARAMETERS=0 lenkey=8 ENDIF IF lenkey=1 RETURN RIGHT(SYS(3),1) ENDIF len1=FLOOR(lenkey/2) len2=lenkey-len1 key1= RIGHT(SYS(3),len1) key2= RIGHT(SYS(2015),len2)

RETURN key1+key2 Theoretically, this method allows more possible keys than FoxPro can store records, without a duplicate; however, there is the possibility of duplication because of the cyclic nature of the algorithm (which is based on the system time and date).

The default 8-character return from UKEY will be sufficient for most systems and the chance of a duplicate is significantly less than 1:10**18. (The actual figure is estimated here: 36**8*10**10). To ensure uniqueness, a SELECT-SQL statement can be used. For example, if you were using a database alias of MYDATA, and a key field of KEYFIELD C(8), you would use the following SELECT-SQL statement: goodkey=.F.  DO WHILE !goodkey newkey= UKEY SELECT KEYFIELD from MYDATA WHERE KEYFIELD=newkey goodkey=IIF(_TALLY=0,.T.,.F.) ENDDO This method will slow down as the data grows. However, this method is adequate for tables with less than 16 million records. For larger tables, use a key length that is greater than 8.

