I am migrating a custom e-commerce site over to Magento. We have well over a hundred thousand products. Many are drop shipped from a number of suppliers. Several suppliers can provide the same product. Naturally they will all have different stock and provide different pricing. We have a custom script that parses all the feeds from all the suppliers and create a single feed that contains the new stock and pricing for all the products. With this volume we couldn't use Magento to import the changes. Thus we are using Magmi. Unfortunately Magmi, from what I can tell, will not zero out the stock if an item isn't in the feed. I could just import the products into another table and do a SQL call to set stock = 0 where the products are not in the new table then run magmi against with the feed file.
First idea: If your only problem is to set all the products to 0 before the import, why don't you do it?
I'm not sure, wether this is a good idea, and wether there are side effects, but importing is often a dirty job, so a SQL before the import might solve the problem.
UPDATE `cataloginventory_stock_item` SET qty = 0;
This means, for the time of the import you don't have any products.
Second idea: If this is a problem, you can think about, building a proxy for the feed, fetching all the products from the shop and add the missing products
I use MAGMI Import. It's a simple, straight-forward solution and most of all, it's fast! It has a simple GUI but it also offers a command-line interface, which makes it easily scriptable.
The learning curve is not so steep, but it requires a lot of attention as it fiddles directly into Magento DB (that's why it's so fast).
I currently import/update about 12.000 SKUs daily in less than 6 minutes.
Hope it helps.
If you want an SQL way of updating instead of EAV way then you can refer to the following blog articles: Updating product qty in Magento in an easier & faster way http://www.blog.magepsycho.com/updating-product-qty-in-magento-in-an-easier-faster-way/
Updating product prices in Magento in an easier & faster way http://www.blog.magepsycho.com/updating-product-prices-in-magento-in-easier-faster-way/
Hope that helps.
There is a free magento extension for bulk updating all product prices from savethemage.com or you can download from Magento Connect here http://www.magentocommerce.com/magento-connect/catalog/product/view/id/17639/s/bulk-update-all-product-prices-4483/ It lets you bulk change all prices in your store by a percentage or flat amount, adding or subtracting.