内部和左外部联接组合 - 两列

Question

刚刚学习 - 感谢您的帮助。

Table1：早餐

Animal	Food
Cow	Grass
Monkey	Banana
Monkey	Apples
Human	Cereal
Croc	Meat

Table 2: 午餐

Animal	Food
Cow	Grass
Monkey	Apples
Human	Sandwich
Croc	Meat

您将如何向 return 所有动物编写查询，但 return NULL 午餐与早餐不同？

预期输出：

Animal	Breakfast	Lunch
Cow	Grass	Grass
Monkey	Banana	NULL
Monkey	Apples	Apples
Human	Cereal	NULL
Croc	Meat	Meat

Answer 1

你会使用 full join:

select coalesce(b.animal, l.animal) as animal,
       b.food as breakfast, l.food as lunch
from breakfast b full join
     lunch l
     on b.animal = l.animal

Answer 2

取决于您使用的 sql 类型：

如果SQLite:

SELECT 
breakfast.animal AS animal, 
breakfast.food AS breakfast, 
CASE WHEN lunch.food=breakfast.food THEN lunch.food ELSE NULL END AS lunch 
FROM breakfast JOIN lunch ON breakfast.animal = lunch.animal;

否则如果MySQL:

SELECT 
breakfast.animal AS animal, 
breakfast.food AS breakfast, 
IF lunch.food=breakfast.food THEN lunch.food ELSE NULL END IF AS lunch 
FROM breakfast JOIN lunch ON breakfast.animal = lunch.animal;

还有其他类型的 SQL，例如 SQL 服务器等，所以我只是猜测它可能是这两种中的任何一种。

Answer 3

这是一个普通的外部连接：

select
   b.animal,
  ,b.food as breakfast
  ,l.food as lunch
from breakfast b 
left join lunch l
  on b.animal = l.animal
 and a.food = b.food

Answer 4

就是一个left join语句，记住left join returns left table中的所有索引，没有配对的用'null'值填充，query看起来像

 select b.Animal, 
    b.Food as Breakfast, 
    l.Food as Lunch
from Breakfast as b
    left join Lunch as l
using(Food);

我们使用 table 中的 Food 列作为索引。取索引 'meat' 然后你让鳄鱼吃肉作为早餐和午餐所以你的输出行是

| croc | meat | meat |

现在取索引 'cereal'，我们知道人类早餐吃麦片，但午餐不吃麦片，然后（当您选择左连接时）值 'Null' 放在午餐栏所以我们有

| human | cereal | null |

作为输出行。

内部和左外部联接组合 - 两列

inner and left outer join combined - with two columns

sql

teradata