[MySQL]多表关联查询技巧

示例表A:

示例表B:

示例表C:

在以上表中执行AB表关联

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

结果

结果出现了2条author_id为1的记录，因为右表中存在了两条关联author_id=1的行

右边出现N条关联左边的记录，结果就会相应出现N条关联了右表出现的记录

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

结果

结果出现了3条author_id=1的记录，因为authors第一次关联了books表book_id为9和12的book关联了author_id为1的作者，而book_id为9的书本则关联了两个orders记录，所以结果集包含3条author_id为1的记录

可以运用

count(),sum()

等函数通过

group by

来统计结果

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

结果集会基于book_id来统计每一本书的订单总额

book_id为9的订单总额为0.80，并且9的记录从多条合并为1条

运用

having

对那些WHERE 关键字无法与合计函数一起使用进行一些筛选查询

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

这时只有sum为0.8的结果被选中

对于组合其他语法查询，也是没问题的

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
ORDER BY prices asc
LIMIT 1,1

多条件join

SELECT `authors`.*, `books`.book_id, `orders`.order_id, 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 AND `orders`.order_date >= `books`.start_date AND `orders`.order_date <= `books`.end_date
GROUP BY `books`.book_id

选取在一定时间区间范围内的order订单，可以看到订单order_id为1的订单不再纳入book_id为9的统计当中，因为它的时间区间不符合join条件