Question

SELECT 
    Recipes.RecipeID, Recipes.RecipeTitle
FROM 
    Recipes 
INNER JOIN
    Recipe_Ingredients ON Recipes.RecipeID = Recipe_Ingredients.RecipeID 
INNER JOIN
    Ingredients ON Ingredients.IngredientID = Recipe_Ingredients.IngredientID
WHERE 
   (Ingredients.IngredientName = 'Beef') 
   AND (Ingredients.IngredientName = 'Garlic')

此SQL查询不返回任何内容。然而，当我单独/分开检查哪里条件而不将它们与AND放在一起时，他们想出了一个名为“烤牛肉”的食谱，它实际上同时含有牛肉和大蒜。因此，它不应该在结果中显示为1行。但它没有显示任何东西。为什么？

Answer 1

它返回NULL，因为一个成分不能同时具有名称。你似乎想要：

SELECT r.RecipeID, r.RecipeTitle
FROM Recipes r INNER JOIN
     Recipe_Ingredients ri
     ON r.RecipeID = ri.RecipeID INNER JOIN
     Ingredients i
     ON i.IngredientID = ri.IngredientID
WHERE i.IngredientName IN ('Beef', 'Garlic')
GROUP BY r.RecipeID, r.RecipeTitle
HAVING COUNT(DISTINCT i.IngredientName) = 2;

Answer 2

AND在行级别运行。对于同一行，IngredientName不能同时为'Beef'和'Garlic'。您可能需要再次加入Ingredients。

Answer 3

如果要为同一列选择2个条件，则不能在同一列中使用AND运算符。

SELECT Recipes.RecipeID, Recipes.RecipeTitle
FROM Recipes INNER JOIN
Recipe_Ingredients ON Recipes.RecipeID = Recipe_Ingredients.RecipeID 
INNER JOIN
Ingredients ON Ingredients.IngredientID = Recipe_Ingredients.IngredientID
WHERE (Ingredients.IngredientName in ('Beef' , 'Garlic') )

这什么都不返回。为什么？

3 个答案: