从多个表中检索数据 - SQL

问题描述：

表搜索：

Date  Product Search_ID 
2017-01-01 Nike   101 
2017-01-01 Reebok   292 
2017-01-01 Nike   103 
2017-01-01 Adidas   385 
2017-01-02 Nike   284

表采购

Date  Product Total_sale 
2017-01-01 Adidas  4 
2017-01-01 Nike   1 
2017-01-01 Adidas  2 
2017-01-02 Nike   3

每件产品可以在同一天之内多行。当天产品的购买总数=总和（total_sale）

我需要找出每件产品每天的购买比率，即购买数量/搜索次数。

为了参考，对于耐克上2017-01-01，搜索的总数是702而购买的总数为47，给我尝试的4 7/702 = 0.0669

购买比率：

select t1.product, sum(t1.Total_sale), count(t2.Search_ID) 
from db.purchases t1 join db.searches 
on t1.date = t2.date and t1.product = t2.product 
where t1.date = '2017-01-01' and t1.product = 'Nike' 
group by t1.product, t1.date 
;

，这给我一个奇怪的结果：

product | sum | count 
----------+-------+------- 
    Nike | 32994 | 32994

......我在做什么错在这里？

答

执行聚集之前的加入：

select p.product, p.sales, s.searches 
from (select p.date, p.product, sum(p.Total_sale) as sales 
     from db.purchases p 
     group by p.date, p.product 
    ) p join 
    (select s.date, s.product, count(*) as searches 
     from db.searches s 
     group by s.date, s.product 
    ) s 
    on p.date = s.date and p.product = s.product 
where p.date = '2017-01-01' and p.product = 'Nike';

注意：您可以移动where纳入子查询，提高性能。这将很容易推广到更多的日子和产品。

答

该联接已经与您的结果集相乘，您将在删除GROUP BY并使用*代替指定的字段时看到它。

select * from db.purchases t1 join db.searches 
on t1.date = t2.date and t1.product = t2.product 
where t1.date = '2017-01-01' and t1.product = 'Nike'

你不需要加入表来计算购买率：

SELECT  
(select sum(t1.Total_sale) from db.purchases t1 where t1.date = '2017-01-01' and t1.product = 'Nike') 
/
(select count(t2.Search_ID) from db.searches t2 where t2.date = '2017-01-01' and t2.product = 'Nike')

答

问题是您要加入两个未汇总的表，因此每个“购买”行都与每个“搜索”行连接。因此，你的结果32994，其中来自702 X 49

正确的方式来实现所期望的结果与加盟将

select t1.product, t1.total_sales, t2.search_count 
from (
      select date, product, sum(total_sales) as total_sales 
      from db.purchases 
      group by date, product 
     ) t1 
join (
      select date, product, count(search_id) as search_count 
      from db.searches 
      group by date, product 
     ) t2 
on  t1.date = t2.date and t1.product = t2.product 
where t1.date = '2017-01-01' and t1.product = 'Nike' 
group by t1.product, t1.date;

从多个表中检索数据 - SQL

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