-- The y_test table is created with two columns: the gid column,
-- which uniquely identifies the row, and the pt1 point column.

CREATE TABLE y_test (gid integer,
                     pt1 ST_Point);

-- The INSERT statements insert two rows.
-- One is a point without a Z coordinate or a measure.
-- The other has both a Z coordinate and a measure.

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

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

-- The query lists the gid column and the double-precision Y coordinate
-- of the points.

SELECT gid, ST_Y(pt1) y_coord 
   FROM y_test;