MySQL Trigger cannot update table – getting ERROR 1442

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The symptom is, that you are running an UPDATE (for all rows) inside a INSERT trigger – both modify the table, which is not allowed.

That said, if I guess the intention of your trigger correctly, you do not want to update all rows, but only the newly inserted row. You can achieve that easily with 

CREATE TRIGGER sum
BEFORE INSERT
ON news
FOR EACH ROW
SET NEW.sum = (NEW.int_views + NEW.ext_views)/NEW.pageviews

Mind that this is a BEFORE INSERT trigger, as you want to change the row before it is written to the table.

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