SQL AVG（）函数返回INT时多列

Question

编辑：我做了一个错误的写作WHERE Users.Username=?代替WHERE Answers.Username=?

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如果我用下面的语句，我得到正确的平均3.33：

public final String SELECT_USER_AND_AVGANSWERS_STMT = 
"SELECT AVG(CAST(Answers.Rating AS FLOAT)) FROM Answers " 
+ "WHERE Answers.Username=?"; 

通知我只是选择AVG。当我尝试选择更多的列时，它返回4，因为它将其四舍五入。

public final String SELECT_USER_AND_AVGANSWERS_STMT = 
"SELECT Users.Username, Users.Nickname, AVG(CAST(Answers.Rating AS FLOAT)) " 
+ "FROM Users, Answers " 
+ "WHERE Users.Username=?" 
+ "GROUP BY Users.Username, Users.Nickname"; 

用户表：

"CREATE TABLE Users(Username VARCHAR(10) PRIMARY KEY," 
+ "Password VARCHAR(8) NOT NULL," 
+ "Nickname VARCHAR(20) NOT NULL," 
+ "Description VARCHAR(50)," 
+ "Photo VARCHAR(4000))"; 

答案表：

"CREATE TABLE Answers(AnswerID INTEGER PRIMARY KEY GENERATED ALWAYS AS IDENTITY," 
+ "QuestionID INTEGER NOT NULL," 
+ "SubmittedTime TIMESTAMP NOT NULL," 
+ "Text VARCHAR(300) NOT NULL," 
+ "Username VARCHAR(10) NOT NULL," 
+ "Rating INTEGER DEFAULT 0 NOT NULL)"; 

使用Java /嵌入式Derby数据库。

感谢您的帮助。

Answer 1

正如您所指出的那样，您尝试在此查询中匹配Users.Username，但在您的第一个查询中匹配Answers.Username，所以这是一个问题！这应该工作：

SELECT Users.Username, Users.Nickname, AVG(CAST(Answers.Rating AS FLOAT)) 
FROM Users, Answers 
WHERE Answers.Username=? 
GROUP BY Users.Username, Users.Nickname 

别的东西，可能是有问题的是聚集与否CAST之前发生。要检查这一点，你可以使用：

SELECT UserName, NickName, CAST(AVG(AnswersRating) AS FLOAT) 
FROM 
(
    SELECT Users.Username As UserName, Users.Nickname As NickName, CAST(Answers.Rating AS FLOAT) As AnswersRating 
    FROM Users, Answers 
    WHERE Answers.Username=? 
) AS Float_Table 
GROUP BY UserName, NickName