insert, if exist update using trigger - mysql

Question

I want to implement a Viewed system for my website. Here is the structure of my tables:
// table1
id | user_or_ip | post_id | date_time // inserting new row for each viewed
// table2
id | post_id | total_viewed // getting the number of total viewed for each post
Now I need to a trigger for insert/update table2 after insert in table1.
I
think I have to use on duplicate key.

Answer 1

You can do this fairly easily.
With the following 2 example table:-
CREATE TABLE table1
(
id INT NOT NULL AUTO_INCREMENT,
user_or_ip VARCHAR(255),
post_id INT,
date_time DATETIME,
PRIMARY KEY (id)
);
CREATE TABLE table2
(
id INT NOT NULL AUTO_INCREMENT,
post_id INT,
total_viewed INT,
PRIMARY KEY (id),
UNIQUE KEY post_id (post_id)
);
you can use the following trigger on table 1 to calculate the count and insert it to table 2:-
CREATE TRIGGER `trig_1` AFTER INSERT ON `table1`
FOR EACH ROW BEGIN
INSERT INTO table2(post_id, total_viewed)
SELECT post_id, COUNT(*)
FROM table1
WHERE post_id = NEW.post_id
GROUP BY post_id
ON DUPLICATE KEY UPDATE total_viewed = VALUES(total_viewed);
END
Note that if you are certain that there will never be an error you could just insert a count of 1 and set it to total_count + 1 in the ON DUPLICATE KEY clause. But if anything fails that prevents the trigger the counts will be forever wrong for that post_id:-
CREATE TRIGGER `trig_1` AFTER INSERT ON `table1`
FOR EACH ROW BEGIN
INSERT INTO table2(post_id, total_viewed)
VALUES(NEW.post_id, 1)
ON DUPLICATE KEY UPDATE total_viewed = total_viewed + 1;
END
Note also that the sub query to get the count will be more efficient with an index on post_id in table1