留下空結果加入時沒有找到匹配

Question

說我有這三個表：

Table: Baskets 
id | name 
1  Sale 
2  Premium 
3  Standard 
4  Expired 

Table: Fruit 
id | name | basketid 
1  Apples 1 
2  Oranges 2 
3  Grapes 3 
4  Apples 2 
5  Apples 4 

Table: Veggies 
id | name | basketid 
1  Carrots  1 
2  Peas  2 
3  Asparagus 1 

它可能看起來像第二兩個表應該剛剛被一個表叫生產，但在實際情況有是他們成爲不同表格的好理由。如果籃子在水果或素食表中有行，我需要編寫一個返回行的連接。我以爲我可以用兩個左連接，像這樣實現：

Select Baskets.*, fruit.name as fruit, 
veggies.name as veggies 
from Baskets 
left join Fruit on Baskets.id = Fruit.basketid 
left join veggies on Baskets.id = Veggies.basketid 
where Baskets.id = 2; 

但這種說法在我想是空字段返回值。實際輸出：

id | name  | fruit | veggies 
2  Premium Oranges  Peas 
2  Premium Apples  Peas 

輸出我想：

id | name  | fruit | veggies 
2  Premium Oranges  
2  Premium Apples  
2  Premium    Peas 

我怎樣才能做到這一點？

Answer 1

您需要工會的水果和蔬菜的表到一個關係，你加入對抗，一拉：

Select Baskets.*, produce.fruitname as fruit, produce.veggiename as veggies 
from Baskets 
left join (SELECT basketid, name as fruitname, NULL as veggiename 
      FROM fruit 
      UNION 
      SELECT basketid, NULL, name 
      FROM veggies) produce 
     ON baskets.id = produce.basketid 
where Baskets.id = 2; 

Answer 2

這個查詢將返回任何行，如果有什麼籃子：

SELECT baskets.*, fruit.name AS fruit, NULL AS veggie 
FROM baskets 
JOIN fruit 
ON  fruit.basketId = baskets.id 
WHERE baskets.Id = 2 
UNION 
SELECT baskets.*, NULL, veggie.name 
FROM baskets 
JOIN veggies 
ON  veggies.basketId = baskets.id 
WHERE baskets.Id = 2 

這個查詢將返回一行有兩個NULLS如果沒有在籃下：

SELECT baskets.*, fruit.name AS fruit, NULL AS veggie 
FROM baskets 
LEFT JOIN 
     fruit 
ON  fruit.basketId = baskets.id 
WHERE baskets.Id = 2 
UNION 
SELECT baskets.*, NULL, veggie.name 
FROM baskets 
LEFT JOIN 
     veggies 
ON  veggies.basketId = baskets.id 
WHERE baskets.Id = 2 

這假定fruit.name和veggie.name都不是可空和唯一的。

該查詢是一樣的，但以往可以更高效：

SELECT baskets.*, fruit.name AS fruit, NULL AS veggie 
FROM baskets 
JOIN fruit 
ON  fruit.basketId = baskets.id 
WHERE baskets.Id = 2 
UNION ALL 
SELECT baskets.*, NULL, veggie.name 
FROM baskets 
JOIN veggies 
ON  veggies.basketId = baskets.id 
WHERE baskets.Id = 2 
UNION ALL 
SELECT baskets.*, NULL, NULL 
FROM baskets 
LEFT JOIN 
     fruit 
ON  fruit.basketId = baskets.id 
LEFT JOIN 
     veggies 
ON  veggies.basketId = baskets.id 
WHERE baskets.Id = 2 
     AND fruit.basketId IS NULL 
     AND veggies.basketId IS NULL 

，並且不承擔任何事情。