CONSTRAINT to check values from a remotely related table (via join etc.)

CHECK constraints cannot currently reference other tables. The manual:

Currently, CHECK expressions cannot contain subqueries nor refer tovariables other than columns of the current row.

One way is to use a trigger like demonstrated by @Wolph.

A clean solution without triggers: add redundant columns and include them in FOREIGN KEY constraints, which are the first choice to enforce referential integrity. Related answer on dba.SE with detailed instructions:

• Enforcing constraints “two tables away”

Another option would be to "fake" an IMMUTABLE function doing the check and use that in a CHECK constraint. Postgres will allow this, but be aware of possible caveats. Best make that a NOT VALID constraint. See:

• Disable all constraints and table checks while restoring a dump

A CHECK constraint is not an option if you need joins. You can create a trigger which raises an error instead.

Have a look at this example: http://www.postgresql.org/docs/9.1/static/plpgsql-trigger.html#PLPGSQL-TRIGGER-EXAMPLE

CREATE TABLE emp (    empname text,    salary integer,    last_date timestamp,    last_user text);CREATE FUNCTION emp_stamp() RETURNS trigger AS $emp_stamp$    BEGIN        -- Check that empname and salary are given        IF NEW.empname IS NULL THEN            RAISE EXCEPTION 'empname cannot be null';        END IF;        IF NEW.salary IS NULL THEN            RAISE EXCEPTION '% cannot have null salary', NEW.empname;        END IF;        -- Who works for us when she must pay for it?        IF NEW.salary < 0 THEN            RAISE EXCEPTION '% cannot have a negative salary', NEW.empname;        END IF;        -- Remember who changed the payroll when        NEW.last_date := current_timestamp;        NEW.last_user := current_user;        RETURN NEW;    END;$emp_stamp$ LANGUAGE plpgsql;CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp    FOR EACH ROW EXECUTE PROCEDURE emp_stamp();

...i did it so (nazwa=user name, firma = company name) :

CREATE TABLE users(  id bigserial  CONSTRAINT firstkey PRIMARY KEY,  nazwa character varying(20),  firma character varying(50));CREATE TABLE test(  id bigserial  CONSTRAINT firstkey PRIMARY KEY,  firma character varying(50),  towar character varying(20),  nazwisko character varying(20));ALTER TABLE public.test ENABLE ROW LEVEL SECURITY;CREATE OR REPLACE FUNCTION whoIAM3() RETURNS varchar(50) as $$declare    result varchar(50);   BEGIN select into result users.firma from users where users.nazwa = current_user;    return result;    END;    $$ LANGUAGE plpgsql;CREATE POLICY user_policy ON public.test    USING (firma = whoIAM3());CREATE FUNCTION test_trigger_function()RETURNS trigger AS $$BEGIN  NEW.firma:=whoIam3();return NEW;END$$ LANGUAGE 'plpgsql'CREATE TRIGGER test_trigger_insert BEFORE INSERT  ON test FOR EACH ROW EXECUTE PROCEDURE  test_trigger_function();