內連接
image.png
-- 通過查看圖書信息表和圖書類別表來獲取圖書編號夭咬、圖書名稱、圖書類別
select book_id,book_name,category from bookinfo inner join bookcategory on bookinfo.book_category_id=bookcategory.category_id
外連接
左外連接
image.png
-- 根據(jù)業(yè)務需求揪胃，我們需要查看圖書類別表中的所有類別下都有哪些圖書
SELECT book_id,book_name,category from bookcategory left join bookinfo on bookcategory.category_id = bookinfo.book_category_id
