-- The ring_linestring table is created with two columns:
-- a smallint column, gid, uniquely identifies the rows,
-- while ln1, a ST_LineString column, stores the ST_LineString geometries.

CREATE TABLE ring_linestring (gid smallint,
                              ln1 ST_LineString);

-- The INSERT statements insert three linestrings into the ln1 column.
-- The first row contains a linestring that's not closed and isn't a ring.
-- The second row contains a closed and simple linestring that is a ring.
-- The third row contains a linestring that is closed but not simple because
-- it intersects its own interior. It's also not a ring.

INSERT INTO ring_linestring VALUES( 
   1,
   ST_LineFromText('linestring (10.02 20.01,10.32 23.98,11.92 25.64)', 1000) 
);

INSERT INTO ring_linestring VALUES( 
   2,
   ST_LineFromText('linestring (10.02 20.01,11.92 35.64,25.02 34.15,19.15 33.94, 10.02 20.01)', 1000) 
);

INSERT INTO ring_linestring VALUES( 
   3,
   ST_LineFromText('linestring (15.47 30.12,20.73 22.12,10.83 14.13,16.45 17.24,21.56 13.37,11.23 22.56,19.11 26.78,15.47 30.12)', 1000) 
);

-- The query returns the results of the ST_IsRing function.
-- The first and third rows return false because the linestrings aren't
-- rings, while the second row returns t (TRUE) because it is a ring.

SELECT gid, ST_IsRing(ln1) Is_it_a_ring
   FROM ring_linestring;