sql server nullif 、isnull、 isnumeric 、cast 的函數小例子

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 (* )                --總數
    , count (nullif (1 , 1 ))        --永遠返回0
    , count (a )                --a數量
    , count (b)                --b數量
    , count (distinct a )        --a不重複數量
    , count (isnull (b, c ))        --b或者c不為null數量
    , count (Coalesce (a , b, c ))    --a或者b或者c不為null數量
    , count (nullif (a , b))        --a不等於b的數量
    , count (nullif (isnumeric (cast (a as varchar (38 ))), 0 ))--a是數位數量
from t_test