测试一下你的T-SQL基础知识-count

下面count的返回值是多少？

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;

答案

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

参考链接：https://technet.microsoft.com/en-us/library/ms175997(v=sql.90).aspx