Update fields with multiple values ​from different tables in MySQL
Full example.

1) Create tables and data (code)

CREATE TABLE Product_categories (product_reference VARCHAR(8), category_id INT, subcategories VARCHAR(8));
INSERT INTO Product_categories VALUES
    ('REF.001',1,'1/2'),
    ('REF.002',2,'1/2/3/4');

CREATE TABLE Categories (id INT, name VARCHAR(32));
INSERT INTO Categories VALUES
    (1,'example_category_1'),
    (2,'example_category_2'),
    (3,'example_category_3'),
    (4,'example_category_4');

CREATE TABLE Importer (product_reference VARCHAR(8), category VARCHAR(128));
INSERT INTO Importer (product_reference) VALUES
    ('REF.001'),
    ('REF.002');


2) Check data (console output)

mysql&gt; SELECT * FROM Product_categories;
+-------------------+-------------+---------------+
| product_reference | category_id | subcategories |
+-------------------+-------------+---------------+
| REF.001           |           1 | 1/2           |
| REF.002           |           2 | 1/2/3/4       |
+-------------------+-------------+---------------+
2 rows in set (0.00 sec)

mysql&gt; SELECT * FROM Categories;
+------+--------------------+
| id   | name               |
+------+--------------------+
|    1 | example_category_1 |
|    2 | example_category_2 |
|    3 | example_category_3 |
|    4 | example_category_4 |
+------+--------------------+
4 rows in set (0.00 sec)

mysql&gt; SELECT * FROM Importer;
+-------------------+----------+
| product_reference | category |
+-------------------+----------+
| REF.001           | NULL     |
| REF.002           | NULL     |
+-------------------+----------+
2 rows in set (0.00 sec)


3) Perform update (code)

UPDATE Importer i, (SELECT pc.product_reference,GROUP_CONCAT(c.name ORDER BY n.n SEPARATOR '&gt;') category
                    FROM (SELECT 1 n UNION SELECT 2 UNION SELECT 3 UNION SELECT 4) n 
                    JOIN Product_categories pc
                    JOIN Categories c ON SUBSTRING_INDEX(SUBSTRING_INDEX(CONCAT('',pc.subcategories,'////'),'/',n.n),'/',-1) = c.id
                    GROUP BY pc.product_reference
                   ) d
SET i.category = d.category
WHERE i.product_reference = d.product_reference;


4) Check result (console output)

mysql&gt; SELECT * FROM Importer;
+-------------------+-----------------------------------------------------------------------------+
| product_reference | category                                                                    |
+-------------------+-----------------------------------------------------------------------------+
| REF.001           | example_category_1&gt;example_category_2                                       |
| REF.002           | example_category_1&gt;example_category_2&gt;example_category_3&gt;example_category_4 |
+-------------------+-----------------------------------------------------------------------------+
2 rows in set (0.00 sec)