SQL where syntax error with trigger - mysql

Question

create trigger cal_retweet before insert on T
for each row begin
set NEW.retweet_change = NEW.retweet_count - retweet_count where id_str = NEW.id_str
end
SQL said there is syntax error near "where id_str = NEW.id_str"
My table looks like this. Where id_str is a unique identifier for a specific tweet. Since I am inserting 50 tweets from a single user every minute, there would be many same id_str. What I want to look at is the change of retweet_count every minute. tweeted_at is when the user tweeted, created_at is when this data is inserted into my database. I want to generate retweet_change for each new data inserted into the database compared to the same old tweet (into the column retweet_change). How should I write the trigger?
After reading some of your comments I changed my code to :
create trigger cal_retweet before update on T
for each row
begin
set NEW.retweet_change = NEW.retweet_count - OLD.retweet_count;
end;
There is still syntax error

Answer 1

There are several issues with this trigger.
You have some syntax errors. You need proper semicolons to delimit your statements.
You have a WHERE statement that is out of place (and actually not needed). You are acting on only a single row at a time, you don't have to match on the id_str.
In order to factor in a calculation using an existing value from the row, you need access to the OLD keyword. For that, you need a trigger that happens on UPDATE, not INSERT. On INSERT, the retweet_change is simply the same as retweet_count; you could alter your INSERT statement to fix that problem.
You may need to explicitly add a statement delimiter as per the comments below.
So all together, I think this trigger should look like:
DELIMITER //
CREATE TRIGGER cal_retweet BEFORE UPDATE ON T
FOR EACH ROW
BEGIN
SET NEW.retweet_change = NEW.retweet_count - OLD.retweet_count;
END;//
DELIMITER ;