模仿session_num的MySQL会话变量的值没有增加

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