How to find all the foreign key records that do not have a primary key using a subquery (a query inside a query). Subquerys can produce the same result set as a Join as well the opposite of a join result set 如何利用子查詢找出沒有主關鍵值的資料。子查詢可產生與join相反的結果。 src: [Video 42 - SQL SubQuery](https://thefrugalcomputerguy.com/grouppg.php?ser=15&grp=15)(https://youtu.be/X87P7ADNF_4)