For example, there are two tables:
CREATE TABLE `products` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `price` decimal(10,2) unsigned NOT NULL, `quantity` smallint(5) unsigned NOT NULL, `name` varchar(100) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; CREATE TABLE `products_item` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `product_id` int(10) unsigned NOT NULL, `quantity` smallint(6) unsigned DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
The main query that returns the result:
select p.id, p.quantity, sum(pi.quantity) as pi_quantity from products p left join products_item pi on pi.product_id=p.id group by p.id having p.quantity > sum(pi.quantity)
I launch it in Mysql-workbench as a result I observe the table from 48 lines.
I need to calculate this number. So far the only working option I have available is to do this:
select count(*) from ( select p.quantity from products p left join products_item pi on pi.product_id=p.id group by p.id having p.quantity > sum(pi.quantity) ) t
What are the disadvantages of this approach?
Should I worry if the subquery returns a very large number as a result?
So how do you calculate the number of rows as a result in this case?
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