MySQL set NULL Query not always works

UPDATE `table` 
SET `sent`='1', `taken`='开发者_开发问答1', `free`=NULL
WHERE `owner`='1' AND ISNULL(`sent`) AND !ISNULL(`free`);

Works, but set free to NULL not always works. Any ideas? Thx

Check your triggers.

UPDATE `table` 
SET `sent`='1', `taken`='1', `free`=NULL
WHERE `owner`='1' AND `sent` is null AND `free` is not null;

Execute the following query to check if it returns all the records you request:

SELECT *
FROM `table`
WHERE `owner`='1' AND ISNULL(`sent`) AND !ISNULL(`free`);