How to use ST_DWithin to query features from one table within a distance of a specific feature or features in another table

Question

I'm pretty new with PostGIS. I have a parcel layer and a census block layer in a postgres database, and I would like to be able to query out all census blocks within a specified distance of a specific parcel. I found this example statement, but I'm not sure how to reference the specific parcel in the query, say by parcel ID?

SELECT roads.roadname, pois.poiname
FROM roads INNER JOIN pois 
ON ST_DWithin(roads.geog, pois.geog, 1609);`

This example uses the geography type, but my layers don't have that. They are in EPSG 3857; can I use the geometry type and get accurate results?

Answer 1

Assuming you are using geography type which I am guessing from geog name, query to find all census blocks within 1609 meters ~ 1 mile) of parcel denoted by key '12345'.

SELECT census_blocks.*
   FROM census_blocks INNER JOIN parcels 
      ON ST_DWithin(census_blocks.geog, parcels.geog, 1609)
WHERE parcels.parcel_id = '12345';