The ATAN function returns a value whose tangent is x in radian. The argument y can be omitted. If y is specified, the function calculates the arc tangent value of y/x. The return value is a DOUBLE type.
ATAN ( [y,] x )
SELECT ATAN(1), ATAN(-1), ATAN(1,-1);
atan(1) atan(-1) atan2(1, -1)
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7.853981633974483e-01 -7.853981633974483e-01 2.356194490192345e+00