判断两个值是否不等，是否相等、获取多列的最大值和最小值

用 IS DISTINCT FROM 判断两个值是否不等，用 IS NOT DISTINCT FROM 判断是否相等、用 greatest 获取多列的最大值，用 least 获取多列的最小值

* |
SELECT
  NULL IS DISTINCT
FROM  NULL,
  NULL IS NOT DISTINCT
FROM  NULL,
  greatest(1, 2, 3) as max,
  least(1, 2, 3) as min

SQL查询结果样例：