The ST_Z function

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

CREATE TABLE z_test (gid integer,
                     pt1 ST_Point);

The following 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 z_test VALUES( 
   1, 
   ST_PointFromText('point (10.02 20.01)', 1000) 
);

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

The query retrieves the values in the gid column and the DOUBLE PRECISION Z coordinate of the points. The first row is NULL because the point does not have a Z coordinate:

SELECT gid, ST_Z(pt1) z_coord
  FROM z_test;

        gid        z_coord 

          1               
          2 5.000000000000