is-null.sql

本程序中即有讲义

--在下面的示例中，对于所有预付款少于 $5,000 或者预付款未知（或为 NULL）的书，
--返回它们的书号及预付款。

USE pubs
SELECT title_id, advance
FROM titles
WHERE advance < $5000 OR advance IS NULL 

ORDER BY title_id

--
select * from discounts
select * from discounts where stor_id  is null
select * from discounts where stor_id =''