Question

我有两张桌子：

食谱

RecipeId | UserId |名称|等

和

RecipeRatings

RecipeRatingId | RecipeId | UserId |评级|评论

我想选择食谱列表并包含平均评分。像这样的东西会起作用：

SELECT
    Recipes.RecipeId,
    Name,
    AVG(RecipeRatings.Rating) AS AverageRating
FROM Recipes
INNER JOIN RecipeRatings
ON RecipeRatings.RecipeId = Recipes.RecipeId
GROUP BY Recipes.RecipeId, Recipes.Name
ORDER BY AverageRating DESC

但我真正喜欢的是获得平均值或当前用户评分。似乎我不应该打扰显示平均值，如果用户自己评价更高或更低。

所以给定一个参数@userId，有没有办法选择食谱列表和当前用户评级（如果有），否则RecipeRatings表中的平均值（如果有的话）？

使用AverageRating和MyRating列也可以，并且将由客户端正确显示它。

Answer 1

你可以试试这个：

SELECT
    R.RecipeId,
    R.Name,
    AVG(RR.Rating) AS AverageRating,
    MAX(CASE WHEN RR.UserId = @userId THEN RR.Rating END) AS MyRating
FROM Recipes AS R
LEFT JOIN RecipeRatings AS RR
ON RR.RecipeId = R.RecipeId
GROUP BY R.RecipeId, R.Name
ORDER BY AverageRating DESC

如果您想显示MyRating，如果它不为空，否则AverageRating您可以查看COALESCE功能。

COALESCE(MAX(CASE WHEN RR.UserId = @userId THEN RR.Rating END),
         AVG(RR.Rating) AS ActualRating

我还将您的INNER JOIN更改为LEFT JOIN，以便结果中仍然可以显示没有任何评级的食谱。

SQL Query选择用户的个人评级，或者如果不存在平均评级

1 个答案: