测试一下你的T-SQL基础知识-count

CREATE TABLE mytable ( myid INT, mychar VARCHAR(10) );
GO
INSERT mytable
( myid, mychar )
VALUES
( 1, 'A' ),
( 2 , 'B'),
( NULL, 'C' ),
( 4, 'D' ),
( NULL, 'E' ),
( 6, 'F' );
--Query1
SELECT COUNT(MyID) FROM mytable;
--Query2
SELECT COUNT( DISTINCT MyID) FROM mytable;
--Query3
SELECT COUNT(DISTINCT *) FROM mytable;

--Query1:4
--Query2:4
--Query3:ERROR