The ST_Y() function

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

CREATE TABLE y_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 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 retrieves the values in the gid column and the DOUBLE PRECISION Y coordinate of the points:

SELECT gid, ST_Y(pt1) y_coord 
   FROM y_test;

        gid        y_coord 

          1 20.01000000000
          2 20.01000000000