Select Duplicate Points PostGIS

Question

I have a table made of merged point datasets in PostGIS. I want to select all of the features where two or more points have identical geom values.

At present I just want to select so I can review the associated field values.

Answer 1

Not all the details are clear, but my general approach would be along these lines:

SELECT *
FROM thetable AS a, thetable AS b
WHERE ST_Equals(a.geom, b.geom) AND a.id <> b.id

Answer 2

Something like this:

SELECT p1.id, p1.name, p1.<other attrib>, p2.id, p2.name, p2.<other attrib> 
FROM points p1, points p2
WHERE ST_Intersects(p1.geom, p2.geom) AND
p1.id <> p2.id;

Some notes:

• With a large number of points you'll probably want to implement a spatial index on the table.
• Also this will find only points that exactly match. Sometimes very small rounding errors can create pairs of points which are essentially overlapping, but the coordinates are different by 0.1 mm or so, and the above will not find that pair. So perhaps wrapping one of the points with a small buffer, or running ST_Dwithin() with a small tolerance might be better.