带条件的 SQL Sum() 函数

Question

这在 SQL 中是如何工作的

sum(o.product_name='Bread') > 0

， 例如，如果订单表 (o) 具有 Product_name 列，使得条目为 Bread，则条件将

o.product_name='Bread'

将 1 返回给 sum 函数，然后 sum 函数会将所有这些相加？

问题是：有2桌顾客，下单。查找购买面包和牛奶但不购买鸡蛋的顾客

Select  c.customer_id , c.customer_name
        FROM customers as c 
        left join
           orders as o
           on c.customer_id = o.customer_id
        group by c.customer_id , c.customer_name
        having 
        sum(o.product_name='Bread') > 0 
        and sum(o.product_name='Milk') > 0
        and sum(o.product_name='Eggs') = 0
        order by c.customer_name

Select  c.customer_id , c.customer_name
        FROM customers as c 
        left join
           orders as o
           on c.customer_id = o.customer_id
        group by c.customer_id , c.customer_name
        having 
        sum(o.product_name='Bread') > 0 
        and sum(o.product_name='Milk') > 0
        and sum(o.product_name='Eggs') = 0
        order by c.customer_name

Answer 1

sum

函数只是统计每个客户的product_name分别为牛奶、面包、鸡蛋的订单记录总数