笛卡尔积介绍
笛卡尔乘积是指在数学中,两个集合X和Y的笛卡尓积(Cartesian product),又称直积,表示为X ×Y,第一个对象是X的成员而第二个对象是Y的所有可能有序对的其中一个成员
准备数据:
create table A(A_ID int primary key auto_increment,A_NAME varchar(20) not null);insert into A values(1,'apple');insert into A values(2,'orange');insert into A values(3,'banana');create table B(A_ID int primary key auto_increment,B_PRICE double);insert into B values(1,2.30);insert into B values(2,3.50);insert into B values(4,null);
展示效果:
mysql> select * from A,B; +‐‐‐‐‐‐+‐‐‐‐‐‐‐‐+‐‐‐‐‐‐+‐‐‐‐‐‐‐‐‐+ | A_ID | A_NAME | A_ID | B_PRICE | +‐‐‐‐‐‐+‐‐‐‐‐‐‐‐+‐‐‐‐‐‐+‐‐‐‐‐‐‐‐‐+ | 1 | apple | 1 | 2.3 | | 2 | orange | 1 | 2.3 | | 3 | banana | 1 | 2.3 | | 1 | apple | 2 | 3.5 | | 2 | orange | 2 | 3.5 | | 3 | banana | 2 | 3.5 | | 1 | apple | 4 | NULL | | 2 | orange | 4 | NULL | | 3 | banana | 4 | NULL | +‐‐‐‐‐‐+‐‐‐‐‐‐‐‐+‐‐‐‐‐‐+‐‐‐‐‐‐‐‐‐+