-- The pointn_test table is created with the gid column,
-- which uniquely identifies each row, and the ln1 ST_LineString column.

CREATE TABLE pointn_test (gid integer,
                          ln1 ST_LineString);

-- The INSERT statements insert two linestring values.
-- The first linestring doesn't have Z coordinates or measures,
-- while the second linestring has both.

INSERT INTO pointn_test VALUES( 
   1, 
   ST_LineFromText('linestring (10.02 20.01,23.73 21.92,30.10 40.23)',1000) 
);

INSERT INTO pointn_test VALUES( 
   2, 
   ST_LineFromText('linestring zm (10.02 20.01 5.0 7.0,23.73 21.92 6.5 7.1,30.10 40.23 6.9 7.2)',1000) 
);

-- The query lists the gid column and the second vertex of each linestring.
-- The first row results in a ST_Point without a Z coordinate or measure,
-- while the second row results in a ST_Point with a Z coordinate and a
-- measure. The ST_PointN function will also include a Z coordinate or
-- measure value if they exist in the source linestring.

SELECT gid, ST_PointN(ln1,2) the_2nd_vertex 
   FROM pointn_test;