MSSQL一個關於Count函數的小實例

--創建測試表
if object_id(N'T_Test',N'U') is null     
  CREATE TABLE [dbo].[T_Test] (
  [ID] int IDENTITY(1, 1) PRIMARY key NOT NULL,
  [Grouping] varchar(50) NOT NULL,
  [Ret]   varchar(10) null,
  [Chk]   varchar(10) NULL
  )
GO
--插入數據
insert into T_Test values('A', 1, 'XXX')
insert into T_Test values('A', 2, 'XXX')
insert into T_Test values('A', 2, 'YYY')
insert into T_Test values('A', 2, null )
insert into T_Test values('A', 2, '' )
insert into T_Test values('A', null, 'cc' )
insert into T_Test values('A', '', 'cc' )
insert into T_Test values('B', 1, 'YYY')
insert into T_Test values('B', 3, 'XXX')
insert into T_Test values('B', 2, 'XXX')
insert into T_Test values('B', 4, null )
insert into T_Test values('B', 5, '' )
insert into T_Test values('B', null, 'cc' )
insert into T_Test values('B', '', 'cc' )

--1、根據Grouping欄位分組 統計欄位Ret(不為空)不同值的數量,
--2、根據Grouping欄位分組 chk不為空的情況下，統計欄位Ret(不為空)不同值的數量,
--主要利用count函數不統計NULL數量，nullif函數
select grouping, 
       --count(distinct case when isnull(ret, '')<>'' then ret else null end),
       count(distinct NULLIF(ret, '')),
       count(distinct case when isnull(chk, '')<>'' then NULLIF(ret, '') else null end)
from t_test group by GROUPING