Question

结构表显示here

如果我使用查询：

select idn
from balans as outerB WHERE idn!='' group by idn order by 
ifnull((select sum(innerB.amount) from balans as innerB 
   where innerB.idn = outerB.idn 
   and status='up'), 0) -
ifnull((select sum(innerB.amount) from balans as innerB 
   where innerB.idn = outerB.idn 
   and status='down'), 0) desc
limit 15

我得到2行。

但是如果我添加条件>0：

select idn from Balans as outerB WHERE idn!='' AND
(
(select sum(innerB.amount) from Balans as innerB 
   where innerB.idn = outerB.idn 
   and type='up') -
(select sum(innerB.amount) from Balans as innerB 
   where innerB.idn = outerB.idn 
   and type='down')
) > 0
group by idn order by 
ifnull((select sum(innerB.amount) from Balans as innerB 
   where innerB.idn = outerB.idn 
   and type='up'), 0) -
ifnull((select sum(innerB.amount) from Balans as innerB 
   where innerB.idn = outerB.idn 
   and type='down'), 0) DESC
limit 15

结果我得到0行......

请告诉我哪里有错误？我为什么得到0行？

Answer 1

您未在IFNULL()条件中加入WHERE。这就是你获得0行而不是2行的原因。子查询返回的总和是NULL（这是SUM()在没有匹配行时返回的内容，尽管人们可能认为{ {1}}更符合逻辑，它返回0。）

这个附加条件，我将它放在NULL子句中，而不是HAVING（在这种情况下对结果没有影响，但它可能更有效）：

WHERE

我没有理由用相关子查询编写此查询。您可以使用派生表和简单连接（还将select idn from Balans as outerB WHERE idn !='' group by idn HAVING ifnull((select sum(innerB.amount) from Balans as innerB where innerB.idn = outerB.idn and type='up'), 0) - ifnull((select sum(innerB.amount) from Balans as innerB where innerB.idn = outerB.idn and type='down'), 0) > 0 order by ifnull((select sum(innerB.amount) from Balans as innerB where innerB.idn = outerB.idn and type='up'), 0) - ifnull((select sum(innerB.amount) from Balans as innerB where innerB.idn = outerB.idn and type='down'), 0) DESC limit 15 ;更正为type）：

status

在 SQL-Fiddle

进行测试

Answer 2

可能是因为

 (select sum(innerB.amount) from Balans as innerB 
  where innerB.idn = outerB.idn 
  and type='up')

在所有情况下都不是一个数字，让MySQL感到困惑。

根据建议，我会添加 IFNULL（..）以防止MySQL认为你在非数字上做 - 。另一种选择是在（选择总和...）之前做 0 + ... ，这也应该告诉MySQL我们在这里谈论数字。

为什么我的查询不返回任何行

2 个答案: