如果我理解正确，你想在一个列中计数所有NULL和所有NOT NULL…

如果是正确的:

SELECT count(*) FROM us WHERE a IS NULL 
UNION ALL
SELECT count(*) FROM us WHERE a IS NOT NULL

阅读评论后，编辑了完整的查询:]

SELECT COUNT(*), 'null_tally' AS narrative 
  FROM us 
 WHERE a IS NULL 
UNION
SELECT COUNT(*), 'not_null_tally' AS narrative 
  FROM us 
 WHERE a IS NOT NULL;

我通常用这个技巧

select sum(case when a is null then 0 else 1 end) as count_notnull,
       sum(case when a is null then 1 else 0 end) as count_null
from tab
group by a

为了提供另一种选择，Postgres 9.4+允许对聚合应用FILTER:

SELECT
  COUNT(*) FILTER (WHERE a IS NULL) count_nulls,
  COUNT(*) FILTER (WHERE a IS NOT NULL) count_not_nulls
FROM us;

SQLFiddle: http://sqlfiddle.com/ # !17/80a24/5

在阿尔贝托的基础上，我添加了汇总。

 SELECT [Narrative] = CASE 
 WHEN [Narrative] IS NULL THEN 'count_total' ELSE    [Narrative] END
,[Count]=SUM([Count]) FROM (SELECT COUNT(*) [Count], 'count_nulls' AS [Narrative]  
FROM [CrmDW].[CRM].[User]  
WHERE [EmployeeID] IS NULL 
UNION
SELECT COUNT(*), 'count_not_nulls ' AS narrative 
FROM [CrmDW].[CRM].[User] 
WHERE [EmployeeID] IS NOT NULL) S 
GROUP BY [Narrative] WITH CUBE;

a为空的元素个数:

select count(a) from us where a is null;

a不为空的元素个数:

select count(a) from us where a is not null;

这有点棘手。假设表只有一列，那么Count(1)和Count(*)将给出不同的值。

set nocount on
    declare @table1 table (empid int)
    insert @table1 values (1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(NULL),(11),(12),(NULL),(13),(14);

    select * from @table1
    select COUNT(1) as "COUNT(1)" from @table1
    select COUNT(empid) "Count(empid)" from @table1

查询结果

如图所示，第一个结果显示表有16行。其中两行为NULL。所以当我们使用Count(*)时，查询引擎会计算行数，所以我们得到Count result为16。但是对于Count(empid)，它会对列empid中的非null值进行计数。结果是14。

所以每当我们使用计数(列)，确保我们照顾空值如下所示。

select COUNT(isnull(empid,1)) from @table1

将同时计算NULL和Non-NULL值。

注意:当表由多个列组成时，同样的情况也适用。Count(1)将给出总行数，而不考虑NULL/Non-NULL值。只有在使用Count(column)对列值进行计数时，我们才需要注意NULL值。