Question

在SQL +中，我创建了2个表来存储数据，1个用于交叉引用它们。我们的目标是能够编写一个单一的查询，告诉我哪辆车在哪家经销店，哪家经销商提供哪辆车等等。问题在于，当我查询任何东西时，每辆车都在每个经销店。

drop table car_dealer;
drop table car;
drop table dealer;

create table dealer(
    did     NUMBER PRIMARY KEY,
    dname       VARCHAR2(20) 
);
create table car(
    cid     NUMBER PRIMARY KEY,
    cname       VARCHAR2(20)
);
create table car_dealer(
    cid     NUMBER REFERENCES car(cid),
    did     NUMBER REFERENCES dealer(did),
    CONSTRAINT pk_car_dealer PRIMARY KEY (cid,did)
);

INSERT INTO car
VALUES (1,'Buick Skylark');

INSERT INTO car 
VALUES (2,'H2 Hummer');

INSERT INTO car 
VALUES (3,'Chevy Suburban');

INSERT INTO car 
VALUES (4,'BMW Z3');

INSERT INTO car 
VALUES (5,'BMW 328i');

INSERT INTO car 
VALUES (6,'Jeep Wrangler');

INSERT INTO car
VALUES (7,'Ford Focus');

INSERT INTO car 
VALUES (8,'Range Rover');

INSERT INTO car 
VALUES (9,'Toyota Supra');

INSERT INTO car 
VALUES (10,'Ferrari Testarosa');

INSERT INTO car
VALUES (11,'Jaguar X12');

INSERT INTO car 
VALUES (12,'Ford Fairmont');

INSERT INTO dealer
VALUES (100,'Midnight Movers');

INSERT INTO dealer 
VALUES (200,'Bubbas Imports');

INSERT INTO dealer
VALUES (300,'Import Motors');

INSERT INTO car_dealer (cid,did)
VALUES (1,100);

INSERT INTO car_dealer (cid,did)
VALUES (2,100);

INSERT INTO car_dealer (cid,did)
VALUES (3,100);

INSERT INTO car_dealer (cid,did)
VALUES (4,100);

INSERT INTO car_dealer (cid,did)
VALUES (5,100);

INSERT INTO car_dealer (cid,did)
VALUES (6,100);

INSERT INTO car_dealer (cid,did)
VALUES (1,200);

INSERT INTO car_dealer (cid,did)
VALUES (2,200);

INSERT INTO car_dealer (cid,did)
VALUES (3,200);

INSERT INTO car_dealer (cid,did)
VALUES (4,200);

INSERT INTO car_dealer (cid,did)
VALUES (5,200);

INSERT INTO car_dealer (cid,did)
VALUES (6,200);

INSERT INTO car_dealer (cid,did)
VALUES (7,300);

INSERT INTO car_dealer (cid,did)
VALUES (8,300);

INSERT INTO car_dealer (cid,did)
VALUES (9,300);

INSERT INTO car_dealer (cid,did)
VALUES (10,300);

INSERT INTO car_dealer (cid,did)
VALUES (11,300);

INSERT INTO car_dealer (cid,did)
VALUES (12,300);

select  dname,
    cname
from    dealer,car
where   did = 200;

select  dname,
cname
from    dealer,car
where   cid = 4;

Midnight Movers（100）和Bubba's Imports（200）都应该只有1-6的车，而进口汽车（300）应该只有7-12的车。相反，所有经销商都拥有所有汽车，我想帮助找出原因。

Answer 1

当您未指定连接时会发生这种情况。要获得正确的结果，您应该将查询更改为以下内容：

SELECT 
    d.dname, c.cname
FROM 
    car_dealer cd
JOIN 
    car c ON cd.cid = c.cid
JOIN 
    dealer d ON cd.did = d.did
WHERE 
    cd.did = 200;

SELECT 
    d.dname, c.cname
FROM 
    car_dealer cd
JOIN 
    car c ON cd.cid = c.cid
JOIN 
    dealer d ON cd.did = d.did
WHERE 
    cd.cid = 4;

此处SQL小提琴http://sqlfiddle.com/#!4/98833/4

如何正确链接2个sql表与引用表

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