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Given a "road", represented by a line, that crosses a "state", represented by a polygon:

What is the SQL query I can use to find the segment of the road that is within the state?

A concrete example.

I have a single line representing Route 66 which traverses the USA. I also have polygons for each state. I want to find the segment of Route 66 that lies within Texas.

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To create an intersection object, or for this case the segment of a line that lies within a polygon, you would use the following PostGIS query:

SELECT a.ID, b.ID, ST_Intersection(a.geom, b.geom)
FROM   thePolygons a
       INNER JOIN
       theLines b
       ON ST_Intersects(a.geom, b.geom) = 1

This would most likely be the results you are looking for.

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