sql help:如何一起添加数量？

您想使用SUM函数和GROUP BY子句.

select category.category_name, SUM((equipment.total_stock-equipment.stock_out)) AS Current_Stock, SUM(equipment.stock_out) as stock_out
from EQUIPMENT, CATEGORY 
WHERE EQUIPMENT.CATEGORY_ID = CATEGORY.CATEGORY_ID and category.category_name = 'Power Tools'
GROUP BY Category.Category_Name