ST_Length(ls)

Returns a double-precision number indicating the length of the
LineString or MultiLineString value ls in its associated spatial
reference system. The length of a MultiLineString value is equal to the
sum of the lengths of its elements. If the argument is NULL or an empty
geometry, the return value is NULL.

URL: https://dev.mysql.com/doc/refman/5.7/en/gis-linestring-property-functions.html

