ST_PointAtDistance(ls, distance)

This function takes a LineString geometry and a distance in the range
[0.0, ST_Length(ls)] measured in the unit of the spatial reference
system (SRS) of the LineString, and returns the Point along the
LineString at that distance from its start point. It can be used to
answer questions such as which Point value is 400 meters from the start
of the road described by the geometry argument.

The function is implemented for LineString geometries in all spatial
reference systems, both Cartesian and geographic.

ST_PointAtDistance() handles its arguments as described in the
introduction to this section, with these exceptions:

o If the geometry argument is not a LineString, an
  ER_UNEXPECTED_GEOMETRY_TYPE
  (https://dev.mysql.com/doc/mysql-errors/8.0/en/server-error-reference
  .html#error_er_unexpected_geometry_type) error occurs.

o If the fractional distance argument is outside the range [0.0,
  ST_Length(ls)], an ER_DATA_OUT_OF_RANGE
  (https://dev.mysql.com/doc/mysql-errors/8.0/en/server-error-reference
  .html#error_er_data_out_of_range) error occurs.

URL: https://dev.mysql.com/doc/refman/8.0/en/spatial-operator-functions.html

