ACOS(X)

此函数返回X的反正弦，X值必须-1到1之间范围否则将返回NULL。考虑下面的例子：

SQL> SELECT ACOS(1);
+---------------------------------------------------------+
| ACOS(1)                                                 |
+---------------------------------------------------------+
| 0.000000                                                |
+---------------------------------------------------------+
1 row in set (0.00 sec)