ST_Longitude(p [, new_longitude_val])

With a single argument representing a valid Point object p that has a
geographic spatial reference system (SRS), ST_Longitude() returns the
longitude value of p as a double-precision number.

With the optional second argument representing a valid longitude value,
ST_Longitude() returns a Point object like the first argument with its
longitude equal to the second argument.

ST_Longitude() handles its arguments as described in the introduction
to this section, with the addition that if the Point object is valid
but does not have a geographic SRS, an ER_SRS_NOT_GEOGRAPHIC
(https://dev.mysql.com/doc/mysql-errors/8.0/en/server-error-reference.html
#error_er_srs_not_geographic) error occurs.

URL: https://dev.mysql.com/doc/refman/8.0/en/gis-point-property-functions.html

