mysql中sum(if())使用

先来一个简单的sum:
select sum(qty) as total_qty from inventory_product group by product_id
这样就会统计出所有product的qty.

但是很不幸，我们的系统里面居然有qty为负值。而我只想统计那些正值的qty，加上if function就可以了。 SQL为：
select sum(if(qty > 0, qty, 0)) as total_qty from inventory_product group by product_id
意思是如果qty > 0, 将qty的值累加到total_qty, 否则将0累加到total_qty.

再加强一点：

select
sum( if( qty > 0, qty, 0)) as total_qty   ,
sum( if( qty < 0, 1, 0 )) as negative_qty_count
from inventory_product 
group by product_id