mysql return results from update - mysql

Question

I want to select a bunch of rows from a mysql database and update the viewed attribute of those once selected (this is a kind of 'I have read these' flag).
Initially I did something like this:
update (
select a, b, c
from mytable
where viewed = '0'
)
set viewed = '1';
This selects the rows nicely and updates their viewed attribute as required. But it does not return the selected rows from the subquery.
Is there a clause I can add, or perhaps I need to store the subquery, etc...? I did consider a transaction but I ended up with the same problem. I have not tried a stored procedure...
Please can someone advise / point me in the right direction on how to do what I do above but in addition return the selected tables from the subquery?
Thanks in advance.
Update:
As pointed out by #Barmar, #a_horse_with_no_name, #fancyPants and #George Garchagudashvil...
In MySQL you have to use two statements to select and update, and not a nested statement as in my initial post, if you want to return the selected rows.
e.g.
begin;
select a, b, c
from mytable
where viewed = '0';
update mytable
set viewed = '1'
where viewed = '0';
commit;
thanks guys.

Answer 1

I would create a simple function:
DELIMITER $$
DROP FUNCTION IF EXISTS `mydb`.`updateMytable`$$
CREATE
/*[DEFINER = { user | CURRENT_USER }]*/
FUNCTION `mydb`.`updateMytable`() RETURNS TEXT
BEGIN
SET #updated := '';
UPDATE mytable
SET viewed = 1
WHERE viewed = 0
AND (
SELECT #updated := CONCAT_WS(',', #updated, id)
) != ''
;
RETURN TRIM(LEADING ',' FROM #updated);
END$$
DELIMITER ;
which updates tables and returns concatenated ids.
From php you call this:
SELECT mydb.updateMytable()
and you get ids in a stirng: 1,2,7,54,132 etc...
Update:
my function is returning string containing comma separated ids:
'1,5,7,52,...' these ids are only which would have been updated during the function call,
better php-mysql example would be (you may and would use PDO):
$query = "SELECT mydb.updateMytable()";
$res = mysql_query($query);
$arr = mysql_fetch_array($res);
$ids = explode(',', $arr[0]);
// now you can do whatever you want to do with ids
foreach ($ids as $id)
{
echo "Hoorah: updated $id\n";
}
also remember to change mydb and mytable according to your database names
Final
because you need more complex functionality, simply run two query:
First run:
SELECT a, b, c
FROM mytable
WHERE viewed = 0
Next run:
UPDATE mytable
SET viewed = 1
WHERE viewed = 0