ST_Y(p [, new_y_val])

With a single argument representing a valid Point object p, ST_Y()
returns the Y-coordinate value of p as a double-precision number.The Y
coordinate is considered to refer to the axis that appears second in
the Point spatial reference system (SRS) definition.

With the optional second argument, ST_Y() returns a Point object like
the first argument with its Y coordinate equal to the second argument.
If the Point object has a geographic SRS, the second argument must be
in the proper range for longitude or latitude values.

ST_Y() handles its arguments as described in the introduction to this
section.

URL: https://dev.mysql.com/doc/refman/8.4/en/gis-point-property-functions.html

