自连接的用法

排列：有序对（ordered pair）如<1,2>
组合：无序对(unordered pair) 如{1,2}

name	price
苹果	50
橘子	100

/* 用于获取可重排列的SQL语句 */
SELECT P1.name AS name_1, P2.name AS name_2
  FROM Products P1, Products P2;

  /* 用于获取排列的SQL语句 */
SELECT P1.name AS name_1, P2.name AS name_2
  FROM Products P1, Products P2
 WHERE P1.name <> P2.name;

 /* 用于获取组合的SQL语句 */
SELECT P1.name AS name_1, P2.name AS name_2
FROM Products P1, Products P2
WHERE P1.name > P2.name;