Warning: your code is flawed in a multi-user environment. Two people could run the query at the same time and get the same ID. One of them will fail on the INSERT if the column has a primary or candidate key, which is a best practice for key fields.
My recommendation is to either have the ID be a auto-incrementing integer field (I'm not a fan of them), or even better, create a table of keys. Each record in the table is for a table that gets keys assigned. I use the a structure similar to this:
Structure for: countergenerator.dbf
Database Name: conferencereg.dbc
Long table name: countergenerator
Number of records: 0
Last updated: 11/08/2008
Memo file block size: 64
Code Page: 1252
Table Type: Visual FoxPro Table
Field Name Type Size Nulls Next Step Default
----------------------------------------------------------------------------------------------------------------
1 ccountergenerator_pk Character 36 N guid(36)
2 ckey Character (Binary) 50 Y
3 ivalue Integer 4 Y
4 mnote Memo 4 Y "Automatically created"
5 cuserid Character 30 Y
6 tupdated DateTime 8 Y DATETIME()
Index Tags:
1. Tag Name: PRIMARY
- Type: primary
- Key Expression: ccountergenerator_pk
- Filter: (nothing)
- Order: ascending
- Collate Sequence: machine
2. Tag Name: CKEY
- Type: regular
- Key Expression: lower(ckey)
- Filter: (nothing)
- Order: ascending
- Collate Sequence: machine
Now the code for the stored procedure in the DBC (or in another program) is this:
FUNCTION NextCounter(tcAlias)
LOCAL lcAlias, ;
lnNextValue, ;
lnOldReprocess, ;
lnOldArea
lnOldArea = SELECT()
IF PARAMETERS() < 1
lcAlias = ALIAS()
IF CURSORGETPROP("SOURCETYPE") = DB_SRCLOCALVIEW
*-- Attempt to get base table
lcAlias = LOWER(CURSORGETPROP("TABLES"))
lcAlias = SUBSTR(lcAlias, AT("!", lcAlias) + 1)
ENDIF
ELSE
lcAlias = LOWER(tcAlias)
ENDIF
lnOrderNumber = 0
lnOldReprocess = SET('REPROCESS')
*-- Lock until user presses Esc
SET REPROCESS TO AUTOMATIC
IF !USED("countergenerator")
USE EventManagement!countergenerator IN 0 SHARED ALIAS countergenerator
ENDIF
SELECT countergenerator
IF SEEK(LOWER(lcAlias), "countergenerator", "ckey")
IF RLOCK()
lnNextValue = countergenerator.iValue
REPLACE countergenerator.iValue WITH countergenerator.iValue + 1
UNLOCK
ENDIF
ELSE
* Create the new record with the starting value.
APPEND BLANK IN countergenerator
SCATTER MEMVAR MEMO
m.cKey = LOWER(lcAlias)
m.iValue = 1
m.mNote = "Automatically created by stored procedure."
m.tUpdated = DATETIME()
GATHER MEMVAR MEMO
IF RLOCK()
lnNextValue = countergenerator.iValue
REPLACE countergenerator.iValue WITH countergenerator.iValue + 1
UNLOCK
ENDIF
ENDIF
SELECT (lnOldArea)
SET REPROCESS TO lnOldReprocess
RETURN lnNextValue
ENDFUNC
The RLOCK() ensures there is no contention for the records and is fast enough to not have bottleneck the process. This is way safer than the approach you are currently taking.
Rick Schummer
VFP MVP