这个概念背后的原因是MySQL要求单位关键字是单数，无论英语语法规则如何。如果我们尝试提供 7 天、2 小时等间隔，那么 MySQL 将产生语法错误，如下所示 -
mysql> Select '2017-02-25 05:04:30' + INTERVAL 2 days; ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'days' at line 1
