원본출처 : http://www.brasileiro.net/postgres/cookbook/view-one-recipe.adp?recipe_id=189

Submitted on: 04-24-2001

Description:

Uses the >>> operator as logical implication (called IMP in many systems, including Microsoft Access. The truth table for IMP is

1 T F

T t f

F t t

It can be used for things like 'if condition one is true, condition two must be true; otherwise, it doesn't really matter' I use it frequently as a quick shorthand in CHECK clauses.

Code:

-- author: joel burton (jburton@scw.org)

-- public domain

CREATE FUNCTION "imp" (boolean,boolean) RETURNS boolean AS '

begin

if ($2) or (not $1) then return ''t'';

end if;

return ''f'';

END;

' LANGUAGE 'plpgsql' WITH ( iscachable );

COMMENT ON FUNCTION "imp" ( boolean,boolean ) IS 'Logical implication';

CREATE OPERATOR >>> (PROCEDURE = imp ,

LEFTARG = boolean ,

RIGHTARG = boolean );