MySQL BIGINT UNSIGNED value is out of range - mysql

Question

I have multiple triggers on my database which is updating a statistics table. This table has a BIGINT UNSIGNED column, but on updating the table, I sometimes catch the error: BIGINT UNSIGNED value out of range. I guess the value that is being inserted results in a smaller value then 0, but that seems very strange behavior in my opinion, given the IF check in my statements.
The statement I use:
UPDATE TABLE SET FileSize=774982 WHERE ID=10;
The trigger causing the problem is defined as:
IF (OLD.FileSize <> NEW.FileSize) THEN
INSERT INTO Statistics VALUES('FileSize', 0, 0, 0)
ON DUPLICATE KEY UPDATE Value = IF((Value - OLD.FileSize) < 0, 0, Value - OLD.FileSize);
INSERT INTO Statistics VALUES('FileSize', 0, 0, NEW.FileSize)
ON DUPLICATE KEY UPDATE Value = Value + NEW.FileSize;
END IF
As you can see, I only update the statistics table if the FileSize has really changed (OLD.FileSize <> NEW.FileSize). Besides, I have also added a check to make sure I don't get values below 0 with IF((Value - OLD.FileSize) < 0, 0, Value - OLD.FileSize). Though, the value get's an out of range error and it seems very unlikely that it goes out the upper bound of the BIGINT UNSIGNED, given the fact that the following update statements work? The Error is not always triggered it seems...?
The trigger consists of two statements as you can see. The simple explanation is that the first statement removes the old file size form the statistics table and the second statement is adding the new file size to the table. (This calculates the difference and updates the statistics table correctly)
So, I don't understand how I can get the error: Database error: BIGINT UNSIGNED value is out of range in '(DB.Statistics.Value - OLD.FileSize)' (error code: 1690, State: 22003)
The new and old file sizes are 244662 and 774982 respectively (in bytes)

Answer 1

When you subtract an UNSIGNED value, the result is also UNSIGNED, which means it can't be negative. You're getting the error when you try to calculate Value - OLD.FileSize in the IF() condition.
Instead of subtracting, use a comparison.
INSERT INTO Statistics VALUES('FileSize', 0, 0, 0)
ON DUPLICATE KEY UPDATE Value = IF(Value < OLD.FileSize, 0, Value - OLD.FileSize);