How to convert a table column to another data type   

The USING syntax is particularly handy because it allows you to control how casts are done. Let us suppose you have a text or varchar field that you realize later on should have been an integer and its padded on top of that because it comes from some stupid DBF or mainframe import.

1. If you do something like

   ALTER TABLE ma_tiger ALTER COLUMN fraddl TYPE integer

   You get this rather unhelpful message:
   column "fraddl" cannot be cast to type "pg_catalog.int4"

2. Even if the above did work, you've got some stuff in there you don't really care about - letters and so forth or an empty string. So you want to control how the cast is done anyway

To resolve this issue - lets suppose we write a simple function like this which takes a string value and if it looks like a number, it converts it to a number otherwise it just returns NULL:

CREATE OR REPLACE FUNCTION pc_chartoint(chartoconvert character varying)
  RETURNS integer AS
$BODY$
SELECT CASE WHEN trim($1) SIMILAR TO '[0-9]+' 
        THEN CAST(trim($1) AS integer) 
    ELSE NULL END;

$BODY$
  LANGUAGE 'sql' IMMUTABLE STRICT;

Now with the USING syntax, we can solve this annoying issue with this command.

ALTER TABLE ma_tiger ALTER COLUMN fraddl TYPE integer USING pc_chartoint(fraddl);