SQL Developer-查询返回重复属性

Question

I'm a beginner in Oracle (using SQL Developer) and I decided to go ahead and start a small project just to test myself. I have created 5 tables and inserted values in all of them, but when I run queries it returns me values that are not even supposed to be there so I'm not sure where I'm wrong.

Here is my code:

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');

So after all of this even a simple Query such as

SELECT Actor_fName, Actor_sName, Film_Title
FROM Actors, Film

It will return me all the names and films with repeating values (eg Leonardo DiCaprio name will appear together with "Never Let Me Go" even though he is not in the movie)

Hopefully there is not much I need to edit and I won't waste your time, but I would be very thankful if you responded back with some clues/answers!

Answer 1

Your query is a cross join rather than an inner join.

A cross join will return every row matched with every other row. An inner join will return only the rows that contain the relationship you specify in the on table1.columnid = table2.columnid (or a where clause that defines the relationship).

For example:

Your query

SELECT Actor_fName, Actor_sName, Film_Title
FROM Actors, Film

will return all actor names and all film titles in all possible combinations.

If, instead, you use

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

You'll only get the movies where you've stated that the actor is in the movie.

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