Getting the length of line inside polygon

Question

How can I get the length (brown marked part) of the "Road 7" inside the red polygon?

My start:

SELECT ST_Length(way)
FROM lines
WHERE road = "Road 7" AND ST_Intersects(line, polygon)

with that, I get only the total length. Someone has an idea?

Answer 1

It's necessary to calculate the actual intersection, e.g. http://postgis.net/docs/ST_Intersection.html

SELECT ST_Length(ST_Intersection(line,polygon)) 

Answer 2

An alternavite method for the mulitpoints in polygon

 SELECT
    sum(ST_LENGTH(ST_Intersection(tr.geom, pd.geom), false)/1000) as km
    FROM roads as tr,
    polygon as pd
    WHERE ST_Intersects(pd.geom, tr.geom);