Question

我是Oracle的初学者（使用SQL Developer），我决定继续开始一个小项目来测试自己。我创建了5个表并在所有表中插入了值，但是当我运行查询时，它返回的值甚至不应该存在，所以我不确定我错在哪里。

这是我的代码：

CREATE table Directors
(

Director_ID NUMBER(10) NOT NULL PRIMARY KEY, 
Genre_ID    NUMBER(10),
Director_fName  VARCHAR2(20) NOT NULL,
Director_lName  VARCHAR2(20) NOT NULL,
FOREIGN KEY     (Genre_ID) REFERENCES Details (Genre_ID)

);

CREATE table Actors
(

Actor_ID    NUMBER(10) NOT NULL PRIMARY KEY,
Film_ID     NUMBER(10),
Actor_fName VARCHAR2(20) NOT NULL,
Actor_lName VARCHAR2(20) NOT NULL,
FOREIGN KEY     (Film_ID) REFERENCES Film (Film_ID)


);

CREATE table Film
(

Film_ID     NUMBER(10) NOT NULL PRIMARY KEY,
Actor_ID        NUMBER(10),
Film_Len    CHAR(5) NOT NULL,
Film_Title  VARCHAR2(30) NOT NULL,
FOREIGN KEY     (Actor_ID) REFERENCES Actors (Actor_ID)


);

CREATE table Details
(

Genre_ID    NUMBER(10) NOT NULL PRIMARY KEY,
Director_ID     NUMBER(10),
Genre       VARCHAR2(30) NOT NULL,
Year_Released   DATE,
FOREIGN KEY     (Director_ID) REFERENCES Directors (Director_ID)


);

CREATE table Studio
(

Studio_ID   NUMBER(10) NOT NULL PRIMARY KEY,
Studio_name VARCHAR2(15) NOT NULL,


);


INSERT INTO Directors VALUES(1,'Martin','Scorsese');
INSERT INTO Directors VALUES(2,'Baz','Luhrmann');
INSERT INTO Directors VALUES(3,'Mark','Romanek');

INSERT INTO Actors VALUES(1,'Matthew','McConnaughy');
INSERT INTO Actors VALUES(2,'Leonardo','DiCaprio');
INSERT INTO Actors VALUES(3,'Margot','Robbie');
INSERT INTO Actors VALUES(4,'Joanna','Lumley');
INSERT INTO Actors VALUES(5,'Carey','Mulligan');
INSERT INTO Actors VALUES(6,'Tobey','Maguire');
INSERT INTO Actors VALUES(7,'Joel','Edgerton');

INSERT INTO Film VALUES(1,180,'The Wolf of Wall Street');
INSERT INTO Film VALUES(2,143,'The Great Gatsby');
INSERT INTO Film VALUES(3,103,'Never Let Me Go');

INSERT INTO Details VALUES(1,'Comedy',2013);
INSERT INTO Details VALUES(2,'Romance',2013);
INSERT INTO Details VALUES(3,'Science Fiction',2008);

INSERT INTO Studio VALUES(1,'Paramount');
INSERT INTO Studio VALUES(2,'Warner Bros');
INSERT INTO Studio VALUES(3,'Film4');

所以这毕竟是一个简单的查询，比如

SELECT Actor_fName, Actor_sName, Film_Title
FROM Actors, Film

它将以重复的价值回报我所有的名字和电影（例如Leonardo DiCaprio的名字将与“Never Let Me Go”一起出现，即使他不在电影中）

希望我没有太多需要编辑的东西，我不会浪费你的时间，但如果你回复一些线索/答案我会非常感激！

Answer 1

您的查询是交叉联接而不是内部联接。

交叉连接将返回与每隔一行匹配的每一行。内部联接将仅返回包含您在on table1.columnid = table2.columnid（或定义关系的where子句）中指定的关系的行。

例如：

您的查询

SELECT Actor_fName, Actor_sName, Film_Title
FROM Actors, Film

将以所有可能的组合返回所有演员姓名和所有电影片名。

相反，如果您使用

SELECT Actor_fName, Actor_sName, Film_Title
FROM Actors INNER JOIN Film 
ON Film.Actor_ID = Actors.Actor_ID

你只会在你所说的演员在电影中的地方看电影。

Here's a link that might help you understand joins.

Answer 2

您可能想要加入。

SELECT * FROM Film INNER JOIN Actors ON Film.Actor_ID = Actors.Actor_ID

SQL Developer - 查询返回重复属性

2 个答案: