转:http://blog.csdn.net/k8080880/article/details/17392341
mysql> select sum(if(classid=1,1,0)) as blue,sum(if(classid=2,1,0)) as red from article;
+------+------+
| blue | red |
+------+------+
| 221 | 274 |
+------+------+
1 row in set (0.00 sec)
mysql> select count(classid=1 or null) as blue,count(classid=2 or null) as red from article;
+------+-----+
| blue | red |
+------+-----+
| 221 | 274 |
+------+-----+
1 row in set (0.01 sec)
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
