子查询和多表 [英] Subqueries and multi tables

问题描述

我有一个关于我遇到的问题的基本问题:

I have a basic question regarding a problem I faced:

假设我有这些表的模型:

Let's say I have this model with these tables :

Food(Quantity, Animal_id)
Race(Race_code, Race_name)
Animal(Animal_id, Race_code)

我被要求通过选择查询找到每个种族的总食用量.

I have been asked to find the total quantity eaten by each race with select query.

(当然是使用SUM函数.显示也需要Race_name)

(Of course by using SUM function. Race_name is also required for the display)

但是我不知道将这些表的属性链接起来从数量到种族名称(我只知道我的推理会是这样的Quantity->animal_id->race_code->;race_name).有什么帮助吗?

But I don't know to link the attributes of these tables to go from the quantity to the race name (I only know that my reasoning will be like this Quanity->animal_id->race_code->race_name). Any help ?

推荐答案

看起来像加入，不是吗?未聚合的列(在本例中为 race_name)必须放入 group by 子句中.

Looks like a join, doesn't it? Columns that aren't aggregated (race_name in this case) have to be put into the group by clause.

select r.race_name,
      sum(f.quantity) sum_quantity
from race r join animal a on a.race_code = r.race_code
            join food f on f.animal_id = a.animal_id
group by r.race_name;

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