The ACOS function returns an arc cosine value of the argument. That is, it returns a value whose cosine is x in radian. The return value is a DOUBLE type. x must be a value between -1 and 1, inclusive. Otherwise, NULL is returned.
ACOS( x )
SELECT ACOS(1), ACOS(0), ACOS(-1);
acos(1) acos(0) acos(-1)
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0.000000000000000e+00 1.570796326794897e+00 3.141592653589793e+00