postgis - Calculating total line lengths within polygon?

I have basically the same conundrum as Calculating line segment lengths within polygon using ArcGIS Desktop? but I'm working in PostGIS, not ArcGIS. Any pointers?

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

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.