SQL / DB2 - retrieve value and update/increment simultaniously - sql

Question

I'm connecting to a DB2 database and executing SQL statements.
One example of what is being done is:
select field from library/file
[program code line finishes executing]
[increment value by one]
update library/file set field = 'incremented value'
I have a need to immediately update the value while returning the value. Rather than having to wait for the script to complete, and then run a separate UPDATE statement.
The concept of what I would like to do is this:
select field from library/file; update library/file set field = (Current Value + 1); go;
Please note... this is not the common SQL database most would be familiar with, it is a DB2 database on an IBM i.
Thanks!

Answer 1

Consider using a DB2 SEQUENCE to manage the next available number, if this file is simply intended to have a single row storing your counter. That is what a SEQUENCE is designed to do.
To set it up, use a CREATE SEQUENCE statement.
To increment the value and retrieve, use a SEQUENCE reference expression of the form NEXT VALUE FOR sequence-name. To find out what the most recent value was, use the PREVIOUS VALUE FOR sequence-name. These expressions can be used like a regular any column expression, such as in a SELECT or INSERT statement.
Suppose, for example you want to do this for invoice numbers (and maybe your accounting department doesn't want their first invoice number to be 000001, so we will initialize it higher).
CREATE SEQUENCE InvoiceSeq
as decimal (7,0)
start with 27000; -- for example
You could get a number for a new invoice like this:
SELECT NEXT VALUE FOR InvoiceSeq
INTO :myvar
FROM SYSIBM/SYSDUMMY1;
But what is this SYSIBM/SYSDUMMY1 table? We're not really getting anything from table, so why are we pretending to do so? The SELECT needs a FROM-table clause. But since we don't need one, let's use a VALUES INTO statement.
VALUES NEXT VALUE FOR InvoiceSeq
INTO :myvar;
So that has incremented the counter, and put the value into our variable. You could use that value to INSERT into our InvoiceHeaders and InvoiceDetails tables.
Or, you could increment the counter as you write an InvoiceHeader, then use it again when writing the InvoiceDetails.
INSERT INTO InvoiceHeaders
(InvoiceNbr, Customer, InvoiceDate)
VALUES (NEXT VALUE FOR InvoiceSeq, :custnbr, :invdate);
for each invoice detail
INSERT INTO InvoiceDetails
(InvoiceNbr, InvoiceLine, Reason, Fee)
VALUES (PREVIOUS VALUE FOR InvoiceSeq, :line, :itemtxt, :amt);
The PREVIOUS VALUE is local to the particular job, so there should be no risk of another job getting the same number.

Answer 2

update library/file set field = field + 1;
select field from library/file;
[program code line finishes executing]
[increment value by one]
This handles the problem of another app updating the number between the time you fetch it and the time you update it. Update it and then use it. If two apps try to update simultaneously, one will wait.
A SEQUENCE object is designed exactly for this purpose, but if you are forced to keep this 'next ID' file updated, this is how I'd do it. Follow the link in the comment by #Clockwork-Muse for info on the SEQUENCE object, or try this example from V5R4.

Answer 3

His request is like this:
UPDATE sometable
SET somecounter = somecounter + 10,
:returnvar = somecounter + 10;
Updates and retrieves at the same time.
This is possible in MSSQL, In fact I use it alot there,
but DB2 doesnt seem to have this feature.