SQL语言基础中的笛卡尔积

笛卡尔积 * 以笛卡尔积连接的表具有下列特征: 连接条件被忽略。 第一个表中的所有的行与第二个表中的所有行相连接。 如果在WHERE子句中加入条件，那么可以避免笛卡尔积. * Cartesian Product When a join condition is invalid or omitted completely, the result is a Cartesian product in which all combinations of rows will be displayed. All rows in the first table are joined to all rows in the second table. A Cartesian product tends to generate a large number of rows, and its result is rarely useful. You should always include a valid join condition in a WHERE clause, unless you have a specific need to combine all rows from all tables. Instructor Note Cartesian product is useful for some tests when you need to generate a large number of rows to simulate a reasonable amount of data.