I have basically the same conundrum as [Calculating line segment lengths within polygon using ArcGIS Desktop?](https://gis.stackexchange.com/questions/52240/how-do-i-calculate-line-segment-lengths-within-a-polygon) but I'm working in PostGIS, not ArcGIS. Any pointers? 

This query seems to like it should work, just to show the sums:

```sql
SELECT county, ST_LENGTH(ST_Intersection(counties.the_geom,routes.the_geom))
FROM counties, routes
WHERE ST_Intersects(counties.the_geom, routes.the_geom)

```

But I'm not confident that I'm getting it right *and* I can't figure out how to reshape that into an update query.