问题描述
我想了解如何使用 MAX 函数在属性为 NULL 时设置属性的 0 值.例如:
I would like to understand how to set 0 value of the attribute when it is NULL with MAX function. For example:
Name columns:
number - date
Values:
10 - 2012-04-04
11 - 2012-04-04
12 - 2012-04-04
13 - 2012-04-15
14 - 2012-06-21
1 - 2013-07-04
Number 是增量字段，但它在新年到来时将自己设置为 1.但结果:
Number is incremental field, but it has set itself 1 when new year has come. But result of:
SELECT (MAX(number)+1) number WHERE date LIKE "2014%"
是 NULL 而不是 1 因为 MAX(number) 是 NULL 而不是 0
is NULL and not 1 because MAX(number) is NULL and not 0
推荐答案
好吧，因为没有像 2014 年这样的日期，所以您会期望 null，因为没有的最大值实际上是没有任何意义的.
Well, as there is no date like 2014, you would expect null, because the maximum of nothing is actually not anyting.
但是这样做:
COALESCE(MAX(number),0)
这意味着:从下一个列表中获取第一个非空的东西，所以如果你的 max 为空，它会给你 0
Which means: get the first non-null thing from the next list, so if your max is null, it'll give you 0
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