How to calculate the angle of vector of X and Y using the ATAN2() function?

ATAN2(Y, X) is a MySQL built-in function that calculates the angle of a given vector ending at (X, Y). For example:

SELECT ATAN2(1, 0), ATAN2(0, 1), ATAN2(-1, 0);
  -- +--------------------+-------------+---------------------+
  -- | ATAN2(1, 0)        | ATAN2(0, 1) | ATAN2(-1, 0)        |
  -- +--------------------+-------------+---------------------+
  -- | 1.5707963267948966 |           0 | -1.5707963267948966 |
  -- +--------------------+-------------+---------------------+

Reference information of the ATAN2() function:

ATAN2(Y, X): ang
  Returns the arc tangent of the two variables X and Y. It is similar to
  calculating the arc tangent of Y / X, except that the signs of both
  arguments are used to determine the quadrant of the result.

Arguments, return value and availability:
  Y, X: Required. The end point of the input vector.
  ang: Return value. The angle of the input vector.
  Available since MySQL 4.0.

Related MySQL functions:

• TAN() - Tangent Trigonometric Value
• ATAN() - Arctangent Trigonometric Value

⇒ BIN() - Converting Integer to Binary String

⇐ ATAN() - Arctangent Trigonometric Value

⇑ MySQL Functions on Numeric Values

⇑⇑ MySQL Function References