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;

使用ISNULL嵌入函数。

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;

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;

select count(isnull(NullableColumn,-1))

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