Question

In DB2, I need to do a SELECT FROM UPDATE, to put an update + select in a single transaction.
But I need to make sure to update only one record per transaction.

Familiar with the LIMIT clause from MySQL’s UPDATE option

places a limit on the number of rows that can be updated

I looked for something similar in DB2’s UPDATE reference but without success.

How can something similar be achieved in DB2?

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Edit: In my scenario, I have to deliver 1000 coupon codes upon request. I just need to select (any)one that has not been given yet.

Answer 1

The question uses some ambiguous terminology that makes it unclear what needs to be accomplished. Fortunately, DB2 offers robust support for a variety of SQL patterns.

To limit the number of rows that are modified by an UPDATE:

UPDATE 
( SELECT t.column1 FROM someschema.sometable t WHERE ... FETCH FIRST ROW ONLY
) 
SET column1 = 'newvalue';

The UPDATE statement never sees the base table, just the expression that filters it, so you can control which rows are updated.

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To INSERT a limited number of new rows:

INSERT INTO mktg.offeredcoupons( cust_id, coupon_id, offered_on, expires_on )
SELECT c.cust_id, 1234, CURRENT TIMESTAMP, CURRENT TIMESTAMP + 30 DAYS 
FROM mktg.customers c 
LEFT OUTER JOIN mktg.offered_coupons o
ON o.cust_id = c.cust_id
WHERE ....
AND o.cust_id IS NULL
FETCH FIRST 1000 ROWS ONLY;

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This is how DB2 supports SELECT from an UPDATE, INSERT, or DELETE statement:

SELECT column1 FROM NEW TABLE (
    UPDATE ( SELECT column1 FROM someschema.sometable 
             WHERE ... FETCH FIRST ROW ONLY
    ) 
    SET column1 = 'newvalue'
) AS x;

The SELECT will return data from only the modified rows.