ACOS(X)

此函數返回X的反正弦，X值必須-1到1之間範圍否則將返回NULL。考慮下麵的例子：

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