Mysql limit results based on value from column - php

Question

I'm trying to limit the number of results of a query based on another table column. For example, I have a table for products and a config table, like this:
tb_product
id | active | name | value | ...
tb_config
max_product | ...
What I'd like to do is something like this
SELECT
a.name, a.value
FROM
tb_product a,
tb_config b
WHERE a.active = 1
LIMIT b.max_product
But I'm getting errors like #1327 - Undeclared variable: b. Is there a way to achieve this result?
Because currently what I'm doing is doing another query to get just the max_product value and then use it as php variable to limit the results, like this:
$limit = "SELECT max_product FROM tb_config";
SELECT name, value FROM tb_product WHERE ativo = 1 LIMIT $limit

Answer 1

Maybe....
SELECT a.name
, a.value
FROM tb_product a
CROSS JOIN (SELECT #Limit:=(SELECT max_product from tb_config))
WHERE a.active = 1
LIMIT #Limit

Answer 2

With help from #ENargit's answer in Variable LIMIT Clause in MySQL, you can do it using a row count variable.
Assuming the following schema:
CREATE TABLE
tb_product
(
id INT PRIMARY KEY AUTO_INCREMENT,
`name` VARCHAR(10),
`value` VARCHAR(10)
);
INSERT INTO
`tb_product`
(`name`, `value`)
VALUES
('Name1','Value1'),
('Name2','Value2'),
('Name3','Value3'),
('Name4','Value4'),
('Name5','Value5'),
('Name6','Value6'),
('Name7','Value7'),
('Name8','Value8'),
('Name9','Value9'),
('Name10','Value10');
CREATE TABLE
`tbl_config`
(
id INT PRIMARY KEY AUTO_INCREMENT,
`type` VARCHAR(10),
`value` INT
);
INSERT INTO
`tbl_config`
(`type`,`value`)
VALUES
('something',10),
('maxrows',7);
You can reference the config table with a subquery:
SELECT * FROM (
SELECT
tb_product.*,
#rownum := #rownum + 1 AS RowNum
FROM tb_product,
(SELECT #rownum := 0) AS CounterTbl
) AS DataTbl
WHERE
RowNum <= (
SELECT
`value`
FROM
`tbl_config`
WHERE
`type` = 'maxrows'
);
Gives you the first 7 rows (according to the config value). You can obviously extend this to do sorting etc.
SQLFiddle: http://www.sqlfiddle.com/#!9/f789e4/2