sql中的笛卡尔积

CREATE TABLE student( 

id INT(10),

NAME VARCHAR(10),

CONSTRAINT pk_id PRIMARY KEY (id)

);

CREATE TABLE account(

id INT(10),

money DOUBLE,

CONSTRAINT pk_id PRIMARY KEY(id)

);

INSERT INTO student VALUES('1','jack');

INSERT INTO account VALUES('1','1.1');

INSERT INTO account VALUES('2','2.1');

INSERT INTO account VALUES('3','2.1');

INSERT INTO account VALUES('4','2.1');

INSERT INTO account VALUES('5','2.1');

INSERT INTO account VALUES('6','2.1');

#产生笛卡尔积

SELECT * FROM student,account;

解决办法:

1左外链接

  select * from student s left join account acc on s.id=acc.id;

2内链接

 SELECT * FROM student s INNER JOIN account acc ON s.id=acc.id;