PostGIS Join Queries results in Missing MultiPolygons

Question

I'm trying to join two PostGIS table, one of DMAs and another of states.

After getting some help, I ran this query:

CREATE TABLE combined AS
     SELECT nextval('polyseq_1') AS id, 
     b.name AS state_name, 
a.dma_1 AS dma_1, 
CASE WHEN ST_Within(a.geom, b.geom) 
    THEN a.geom 
     ELSE ST_MULTI(ST_INTERSECTION(a.geom,b.geom)) end AS geom 
FROM us_states b 
JOIN us_dma a ON ST_INTERSECTS(a.geom, b.geom)

However, the results I get have some gaps, for instance, it looks like the Dallas DMA in Texas is completely missing. I'm not familiar with PostGIS but I am unable to figure why the query will cause polygons to be missing. A picture is below:

Answer 1

You might want to change your ST_Within(a.geom,b.geom) to ST_CoveredBy(a.geom, b.geom). If a.geom shares a border with b.geom, then ST_Within will return false, but ST_CoveredBy will return true.

Still the best article explaining this subtlety is this - http://lin-ear-th-inking.blogspot.com/2007/06/subtleties-of-ogc-covers-spatial.html