-- The m_test table is created with the gid integer column,
-- which uniquely identifies the row, and the pt1 point column
-- that stores the sample geometry.

CREATE TABLE m_test (gid integer,
                     pt1 ST_Point);

-- The INSERT statements insert a point with measures
-- and a point without measures.

INSERT INTO m_test VALUES( 
   1, 
   ST_PointFromText('point (10.02 20.01)', 1000) 
);

INSERT INTO m_test VALUES( 
   2, 
   ST_PointFromText('point zm (10.02 20.01 5.0 7.0)', 1000) 
);

-- In this query the SE_M function lists the measure values of the points.
-- Because the first point doesn't have measures, the SE_M function returns NULL.

SELECT gid, SE_M(pt1) The_measure
   FROM m_test;