Update mysql database with subquery condition - mysql

Question

I want to write 1 SQL syntax that will place current transaction in group. This transaction had to be done within last 60 seconds.
Grouping current transaction with other existing transactions is done by assigning group id number (GRID) that is copied from other transaction also performed within a last minute.
In other words:
purchase is done and SQL script will look for other purchases that has been done within last minute and if found it will take group number from found row and assign to current purchase, so in this case every purchase made within a minute will find itself in a group.
This is the update statement below I have composed
UPDATE TRANSACTIONS
SET GRID=(SELECT G FROM
(SELECT GRID AS G
FROM TRANSACTIONS
WHERE CUST_ID='123ID'
AND STAMP+60>UNIX_TIMESTAMP()
LIMIT 1)
AS t),
STAMP=UNIX_TIMESTAMP()
WHERE CUST_ID='123ID'
AND STAMP+60>UNIX_TIMESTAMP();
However this always returns number of updated rows, even if row which exists is only the one is due to be updated, or the other row that was found has no group number assigned yet. Which is obvious as it updates with whatever value was found in subquery. If nothing found it will update with empty value.
There are 2 solutions I am interested in:
I want this script to stop performing update (by condition) if found (from the sub query) value is basically empty.
or
I want to insert condition that if subquery returns empty value, the fix string of characters will be inserted instead.

Answer 1

After a while of exploring my issue I have come to solution.
The following MySQL syntax serves what I want.
Please note very interesting MySQL function IFNULL(). Can be very handy!
UPDATE TRANSACTIONS
SET GRID=(SELECT G FROM
(SELECT IFNULL(GRID, 'NO ID') AS G
FROM TRANSACTIONS
WHERE CUST_ID='123ID'
AND STAMP+60>UNIX_TIMESTAMP()
LIMIT 1)
AS t),
STAMP=UNIX_TIMESTAMP()
WHERE CUST_ID='123ID'
AND STAMP+60>UNIX_TIMESTAMP();