没有注意过的count(0),count(1),count(*),count(列名)

-- 创建测试表
create   table  tb(id  varchar ( 10 ))
-- 插入非空数据
insert  tb  select   ' test '
go
-- 测试
select   count ( 0 )  as   ' count(0) ' , count ( 1 )  as   ' count(1) ' , count ( * )  as   ' count(*) ' , count (id)  as   ' count(id) '   from  tb
-- 结果
/*
count(0)    count(1)    count(*)    count(id)
1                1            1            1
*/
-- 插入null值
insert  tb  values ( null )
go
-- 测试
select   count ( 0 )  as   ' count(0) ' , count ( 1 )  as   ' count(1) ' , count ( * )  as   ' count(*) ' , count (id)  as   ' count(id) '   from  tb
-- 结果
/*
count(0)    count(1)    count(*)    count(id)
2                2            2            1
*/
-- 插入空值
insert  tb  values  ( '' )
go
-- 测试
select   count ( 0 )  as   ' count(0) ' , count ( 1 )  as   ' count(1) ' , count ( * )  as   ' count(*) ' , count (id)  as   ' count(id) '   from  tb
-- 结果
/*
count(0)    count(1)    count(*)    count(id)
3            3            3            2
*/
-- 结论
/*
count(0)=count(1)=count(*) --不忽略null值和空值
count(列名) --忽略null值
*/