NULL is case sensitive. The correct SQL query for getting attribute_id=80 and value is NULL would be:
select * from catalog_product_entity_decimal where attribute_id=80 and value is NULL;
Please note that the attribute_id will differ between Magento installations and can be found with:
SELECT * FROM eav_attribute where attribute_code LIKE '%weight%';
This will not give you what you want as the attribute_id will not exist in the catalog_product_entity_decimal-table if the product is not having a weight set.
One easy way to get products without weight is through the admin-panel.
- Go into your product-catalog, through Catalog ==> Products.
- Press on Columns (cog-wheel icon, on the top left corner of the product table).
- Select to show weight and sort on the weight-column.