MySQL:

ACOS

Syntax:
ACOS(X)

Returns the arc cosine of X, that is, the value whose cosine is X.
Returns NULL if X is not in the range -1 to 1.

mysql> SELECT ACOS(1);
-> 0
mysql> SELECT ACOS(1.0001);
-> NULL
mysql> SELECT ACOS(0);
-> 1.5707963267949

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