这个概念背后的原因是，无论英语语法规则如何，MySQL都要求unit关键字为单数。如果我们尝试提供7天，2小时等间隔，那么MySQL将产生如下语法错误-
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