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解析关于SQL语句Count的一点细节
- WBOYOriginal
- 2016-06-07 16:18:461227browse
if (object_id ('t_test' ) 0 ) drop table t_test go create table t_test (a sql_variant , b sql_variant , c sql_variant ) insert into t_test select 1 , 1 , 'a' insert into t_test select 1 , getdate (), null insert into t_test select 'a' , nu
if (object_id ('t_test' )> 0 )
drop table t_test
go
create table t_test (a sql_variant , b sql_variant , c sql_variant )
insert into t_test select 1 , 1 , 'a'
insert into t_test select 1 , getdate (), null
insert into t_test select 'a' , null , 1
insert into t_test select 3 , null , null
insert into t_test select null , null , null
go
select * from t_test
go
select
count (* )
--总数
, count (nullif (1 , 1 ))
--永远返回0
, count (a )
--a数量
, count (b)
--b数量
, count (distinct a )
--a不重复数量
, count (isnull (b, c ))
--b或者c不为null数量
, count (Coalesce (a , b, c )) --a或者b或者c不为null数量
, count (nullif (a , b))
--a不等于b的数量
, count (nullif (isnumeric (cast (a as varchar (38 ))), 0 ))--a是数字的数量
from t_test