mysql FLOOR(x)返回小于或等于x的最大整数

实例如下：

<code class="language-sql">mysql> SELECT FLOOR(2.3), FLOOR(-2.3);
+------------+-------------+
| FLOOR(2.3) | FLOOR(-2.3) |
+------------+-------------+
|          2 |          -3 |
+------------+-------------+
 row in set (0.00 sec)</code>