MySQL之多表查询练习

Posted 2021-01-15 Charles-MQ

author_id	author_name
1	Kimmy
2	Abel
3	Bill
4	Berton

	bo ok_id	author_id	start_date	end_date
	9	1	2017-09-25 21:16:04	2017-09-25 21:16:06
	10	3
	11	2	2017-09-25 21:21:46	2017-09-25 21:21:47
	12	1
	13	8

 

	order_id	book_id	price	order_date
	1	9	0.2	2017-09-24 21:21:46
	2	9	0.6	2017-09-25 21:16:04
	3	11	0.1	2017-09-25 21:21:46

 

 

 

在以上表中执行ABC表关联

SELECT `authors`.*, `books`.book_id, `orders`.order_id, `orders`.price FROM `authors`
LEFT JOIN `books` ON `authors`.author_id = `books`.author_id
LEFT JOIN `orders` ON `books`.book_id = `orders`.book_id　　　　　　　　

#通过group by 分类，使用sum ,avg,count等函数求值

SELECT `authors`.*, sum(`orders`.price) FROM `authors`
LEFT JOIN `books` ON `authors`.author_id = `books`.author_id
LEFT JOIN `orders` ON `books`.book_id = `orders`.book_id
GROUP BY `books`.book_id

 

#使用where条件，where不能用于选取列的AS别名判断，mysql的处理机制是先进行选取，再进行筛选

SELECT `authors`.*, `books`.book_id, `orders`.order_id, sum(`orders`.price) AS prices FROM `authors`
LEFT JOIN `books` ON `authors`.author_id = `books`.author_id
LEFT JOIN `orders` ON `books`.book_id = `orders`.book_id AND `orders`.order_date >= `books`.start_date AND `orders`.order_date <= `books`.end_date
WHERE `orders`.price is not NULL
GROUP BY `books`.book_id

 

使用having关键字，对于无法使用where字段的price进行统计

SELECT `authors`.*, `books`.book_id, sum(`orders`.price)AS prices FROM `authors`
LEFT JOIN `books` ON `authors`.author_id = `books`.author_id
LEFT JOIN `orders` ON `books`.book_id = `orders`.book_id
GROUP BY `books`.book_id
HAVING prices > 0.1