Mysql session variable emulating row_num does not increase in value - mysql

Question

Since MySql does not provide a row_num facility, I was trying to emulate its functionality using session variables (#variable_name) in my query
the problem statement is as follows
Given a table 'product_view' having columns 'productTitle' and 'categoryName',
select the first 2 products from each category whose title matches a provided string
Going by the following query
SELECT
#row_num := CASE
WHEN #cat_name = categoryName THEN #row_num + 1
ELSE 1
END
AS num,
#cat_name := categoryName AS categoryName,
productTitle
FROM product_search
WHERE
cityId = 1 AND
mainStatus= 'Active' AND
stock_status = 'In Stock' AND
prodQuantity != 0 AND
productTitle LIKE '%apple%'
ORDER BY categoryName, LOCATE('apple', productTitle), productTitle
This is expected to give row_num values 1,2 for category1 and 1,2 for category 2 and so on (wherever ther is a match for 'Apple').
But the rownum value is always stuck at 1
Something similar on the fiddle below
FIDDLE
Can someone please help me out on this

Answer 1

In more recent versions of MySQL, you need to do the order by in a subquery.
In addition, you cannot set #cat_name in one expression and use it in another. MySQL does not guarantee the order of evaluation of expressions in a SELECT. So, you don't know which gets evaluated first, the assignment or the comparison.
I usually write this as:
SELECT (#rn := IF(#cat_name = categoryName, #rn + 1,
IF(#cat_name := categoryName, 1, 1)
)
) as num,
categoryName, productTitle
FROM (SELECT ps.*
FROM product_search ps
WHERE cityId = 1 AND
mainStatus= 'Active' AND
stock_status = 'In Stock' AND
prodQuantity <> 0 AND
productTitle LIKE '%apple%'
ORDER BY categoryName, LOCATE('apple', productTitle), productTitle
) ps CROSS JOIN
(SELECT #cat_name := '', #rn := 0) params