Question

On my MySQL database, I have a unsigned mediumint column, which I keep incrementing and decrementing its value.

However, I want to be sure that it doesn’t decrement when its value is 0. If I do, the UPDATE will make the value to take the maximum value of mediumint, 16777215.

So, I have this query when decrementing:

UPDATE `counters` SET `received`=IF(CAST(`received`-1 AS SIGNED)>0, `received`-1, 0) WHERE `id`="1234"

This was the only trick I got to work with only one query.

It works very well on MySQL 5.1, but, yesterday I upgraded to MySQL 5.5 and now I get these errors:

BIGINT UNSIGNED value is out of range in '(`db_main`.`counters`.`received` - 1)'

Is there any way to fix this?
Or… is there any other way to make sure, in only one query, that it won’t take the value of “16777215” when subtracting 1 to 0?

EDIT – I could use the WHERE, adding ” AND received > 0″. The thing is that sometimes I want to update two columns at the same time, and if I put all the columns > 0 in the WHERE clause, it won’t update ANY of the columns.

Thank you.

Answer 1

That seems a little … torturous, with all that casting and conditional stuff (as an aside, per-row functions rarely scale well in big tables).

Why don’t you just modify the where clause to stop the decrment if it’s already zero?

update counters set received = received - 1
    where id = '1234' and received > 0

---

If there are other columns you want updated irrespective of the value of received, you can do that with a transaction:

begin transaction
update counters set received = received - 1 where id = '1234' and received > 0
update counters set xyzzy = 'plugh' where id = '1234'
commit transaction

This will decrement received, but not below zero, and it will change xyzzy no matter what. And it still keeps away from the per-row stuff that brings down many queries.