INNER JOIN 语句中如何使用括号

Question

我似乎无法弄清楚如何在innerjoin 语句中正确使用括号。例如，当该语句中总共执行了 4 个 INNERJOINS 时，为什么需要在 FROM 子句后面放置 3 个括号。

SELECT Recipe_Classes.RecipeClassDescription,Recipes.RecipeTitle,Recipes.Preparation, Ingredients.IngredientName, Recipe_Ingredients.RecipeSeqNo,   Recipe_Ingredients.Amount, Measurements.MeasurementDescription 
FROM (((Recipe_Classes 
INNER JOIN Recipes 
ON Recipe_Classes.RecipeClassID = Recipes.RecipeClassID) 
INNER JOIN Recipe_Ingredients 
ON Recipes.RecipeID = Recipe_Ingredients.RecipeID) 
INNER JOIN Ingredients 
ON Ingredients.IngredientID = Recipe_Ingredients.IngredientID) 
INNER JOIN Measurements 
ON Measurements.MeasureAmountID = Recipe_Ingredients.MeasureAmountID 
ORDER BY RecipeTitle, RecipeSeqNo

Answer 1

不需要括号。但建议使用表别名！

SELECT rc.RecipeClassDescription, Recipes.RecipeTitle, r.Preparation, 
       i.IngredientName, ri.RecipeSeqNo, ri.Amount, m.MeasurementDescription 
FROM Recipe_Classes rc INNER JOIN
     Recipes r
     ON rc.RecipeClassID = r.RecipeClassID INNER JOIN
     Recipe_Ingredients ri
     ON r.RecipeID = ri.RecipeID INNER JOIN
     Ingredients i
     ON i.IngredientID = ri.IngredientID INNER JOIN
     Measurements m
     ON m.MeasureAmountID = ri.MeasureAmountID
ORDER BY RecipeTitle, RecipeSeqNo;

尽管括号在理论上很有用（在某些具有外连接的情况下），但我发现在实践中不需要它们。MS Access 是一个例外，它需要括号来进行多个连接。