PostGIS Cookbook - Second Edition

How it works...

Spatial joins are one of the key features that unleash the spatial power of PostGIS. For a regular join, it is possible to relate entities from two distinct tables using a common field. For a spatial join, it is possible to relate features from two distinct spatial tables using any spatial relationship function, such as ST_Contains, ST_Covers, ST_Crosses, and ST_DWithin.

In the first query, we used the ST_Intersects function to join the earthquake points to their respective state. We grouped the query by the state column to obtain the number of earthquakes in the state.

In the second query, we used the ST_DWithin function to relate each city to the earthquake points within a 200 km distance of it. We filtered out the cities ...