Session 14_Q1

1. ALTER PROCEDURE Get_Sales_Avg @author varchar(11)
AS
if (select count(*) from authors where au_id=@author)=0
raiserror ('Author ID khong dung', 16, 1)
else
select avg(a.ytd_sales) as 'Trung binh year-to-day sale'
from titles a, titleauthor b
where b.title_id=a.title_id and b.au_id=@author