This function returns the arc cosine, in radians, of a given
number. For input greater than 1 or less than –1, NULL is returned.
Here is an example:
SELECT ACOS(.5), ACOS(1.5);
+----------+-----------+
| ACOS(.5) | ACOS(1.5) |
+----------+-----------+
| 1.047198 | NULL |
+----------+-----------+