如何将查询与子查询连接起来？

Posted 2023-05-08

【中文标题】如何将查询与子查询连接起来？

【英文标题】：How to join queries with a subquery?

【发布时间】：2020-09-28 08:39:57

【问题描述】：

所以我是一个完全新手，试图解决这个练习，我必须找到所有标记为素食但成分中含有火鸡肉的菜肴。

这是我尝试过的（这是我内部连接 ​​3 个表以查找成分的地方）：

SELECT Name
FROM Dishes
INNER JOIN DishesIngredients ON DishesIngredients.DishId = s.Id
INNER JOIN Ingredients ON DishesIngredients.IngredientID = Ingredients.ID

这是我似乎无法加入子查询来识别 Vegetarian 标签的地方：

WHERE Ingredients.Name = 'Turkey meat' =
(SELECT Name
FROM Tags
INNER JOIN DishesTags ON DishesTags.TagID = Tags.ID
INNER JOIN Dishes ON DishesTags.DishID = Dishes.ID)

数据库的图在这里供参考：

【参考方案1】：

让我们先来看看有多少菜肴以火鸡肉为原料。

你有：

SELECT D.ID
FROM
Dishes D 
JOIN DishIngredients DI ON D.ID = DI.DishID
JOIN Ingredients I ON DI.IngredientID = I.ID
WHERE I.Name LIKE 'Turkey meat'

然后获取所有带有“素食”标签的菜肴。

SELECT D.ID
FROM
Dishes D 
JOIN DishIngredients DI ON D.ID = DI.DishID
JOIN Ingredients I ON DI.IngredientID = I.ID
JOIN DishesTags DT on D.ID = DT.DishID
JOIN Tags T ON DT.TagID = T.ID

WHERE I.Name LIKE 'Turkey meat'
AND T.Name = 'Vegetarian'

【参考方案2】：

您可以使用exists 和子查询：

select d.*
from dishes d
where
    exists (
        select 1 
        from dishestags dt
        innerjoin tags t on t.id = dt.tagid
        where dt.dishid = d.id and t.name = 'Vegetarian'
    )
    and exists (
        select 1 
        from dishesingredients di
        inner join ingredients i on i.id = di.ingredientid
        where di.dishid = d.id and i.name = 'Turkey'
    )