MySQL 5.6

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CREATE TABLE articles (id INT, image VARCHAR(255));
CREATE TRIGGER tr_name
BEFORE INSERT
ON articles
FOR EACH ROW
SET NEW.image = CONCAT('catalog/blog-image/', COALESCE((SELECT 1 + MAX(SUBSTRING_INDEX(image, '/', -1)) FROM articles), 1), '.jpg');
INSERT INTO articles (id) VALUES (1);
SELECT * FROM articles;


id
image


1
catalog/blog-image/1.jpg


INSERT INTO articles (id) VALUES (2);
SELECT * FROM articles;


id
image


1
catalog/blog-image/1.jpg

2
catalog/blog-image/2.jpg


INSERT INTO articles (id) VALUES (3);
SELECT * FROM articles;


id
image


1
catalog/blog-image/1.jpg

2
catalog/blog-image/2.jpg

3
catalog/blog-image/3.jpg

id	image
1	catalog/blog-image/1.jpg
2	catalog/blog-image/2.jpg
3	catalog/blog-image/3.jpg