#select to view all products without weight in database:
#TAKE CARE that weight attribute id is the right value

SELECT p.sku, w.value FROM catalog_product_entity as p
LEFT JOIN catalog_product_entity_decimal as w ON (p.entity_id = w.entity_id AND w.attribute_id = 101)
WHERE w.value IS NOT NULL;

#insert weight in catalog_product_entity_decimal for all products without value in that table:
#TAKE CARE to use the right entity_ids (depending on magento version)

INSERT INTO catalog_product_entity_decimal (entity_type_id, attribute_id, store_id, entity_id, value)
SELECT 10, 101, 0, p.entity_id, 1.000 FROM catalog_product_entity as p
LEFT JOIN catalog_product_entity_decimal as w ON (p.entity_id = w.entity_id AND w.attribute_id = 101)
WHERE w.value IS NULL;