Joining tables with different value prefix - mysql

Question

I'm trying to join two tables but the problem is that in the second table the value that is the same as in table one has a prefix to it(this tables are generated after opencart instalation - demo data):
Table 1: category
-----------------------------
| category_id | category_name |
|-----------------------------|
| 1 | Components |
| 2 | Laptops |
Table 2: seo_url
------------------------------------------
| seo_url_id | query | keyword |
|------------------------------------------|
| 35 | category_id=1 | components |
| 78 | category_id=2 | laptops |
So the id of a category is in column category_id in Table 1 and it is a number but in Table 2 it is in column query and it has a prefix of category_id= and then the id x(in case of category laptops x being 2).
Can somebody please help me understand how i could join this tables in this situation?
So far i was trying to add category_id= + like this:
SELECT a.id, a.category_name, b.query
FROM category AS a
INNER JOIN seo_url AS b
ON a.category_id = 'category_id=' + b.query
P.S I tried ON 'category_id=' + a.category_id
P.S.S There are also product_id so i don't know if i could use LIKE but i was thinking about it, searched for it and couldn't find a way to make it work.
Thank you! D:

Answer 1

In MySQL, use the function CONCAT(...) that can append strings and numbers, and it is compatible with different versions of that database.
Your fixed query would be:
SELECT a.id, a.category_name, b.query
FROM category AS a
INNER JOIN seo_url AS b
ON CONCAT('category_id=', a.category_id) = b.query;
Your table seo_url already has the 'category_id=' in the values of the field query, so you don't need to append it.
Additionally, I'd recommend you to name the table aliases with more representative names, instead of using a and b.
Hope this helps you to solve your problem!

Answer 2

Use nested REVERSE functions with + 0 to autocast "parse" out the integer.
Query
SELECT
REVERSE(REVERSE('category_id=2') + 0)
UNION ALL
SELECT
REVERSE(REVERSE('category_id=21') + 0)
Result
| REVERSE(REVERSE('category_id=2') + 0) |
|---------------------------------------|
| 2 |
| 21 |
see demo http://sqlfiddle.com/#!9/340e01/530
Use it in your query.
Query
SELECT a.category_id, a.category_name, b.query
FROM category AS a
INNER JOIN seo_url AS b
ON a.category_id = REVERSE(REVERSE(b.query) + 0)
Result
| category_id | category_name | query |
|-------------|---------------|---------------|
| 1 | Components | category_id=1 |
| 2 | Laptops | category_id=2 |
see demo http://sqlfiddle.com/#!9/ef5781/1