SQL Server 2005 - 检查Null DateTime值

Question

我正在尝试计算具有null DateTime值的记录数。但是，出于某种原因，我的尝试失败了。我没有运气就尝试了以下两个查询：

SELECT COUNT(BirthDate) 
  FROM Person p
 WHERE p.BirthDate IS NULL

和

SELECT COUNT(BirthDate)
  FROM Person p
 WHERE p.BirthDate = NULL

我做错了什么？当我查询所有记录时，我可以看到BirthDate为NULL的记录。

Answer 1

SELECT COUNT(*)
FROM Person
WHERE BirthDate IS NULL

Answer 2

所有答案都是正确的，但我会解释原因......

COUNT(column) ignores NULLs, COUNT(*) includes NULLs.

所以这有效......

SELECT COUNT(*)
FROM Person
WHERE BirthDate IS NULL

Answer 3

试试这个

SELECT     COUNT(*) 
FROM     Person p
WHERE     p.BirthDate IS NULL

Answer 4

这种情况正在发生，因为您尝试在NULL上执行COUNT。我认为，如果您查看邮件标签，则可能会在其中显示NULL values eliminated from aggregate

的消息

您需要更改的是您正在计算的字段

Select Count (1) FROM Person WHERE BirthDate IS NULL

Select Count (*) FROM Person WHERE BirthDate IS NULL

Select Count (1/0) FROM Person WHERE BirthDate IS NULL

Select Count ('duh') FROM Person WHERE BirthDate IS NULL /* some non null string*/

Answer 5

您需要使用“IS NULL”而不是“= NULL”

SELECT
  COUNT('')
FROM
  Person p
WHERE
  BirthDate IS NULL