Обсуждение: order and order line item

Suppose we have the usual order & order line item entities:
CREATE TABLE "order" (  id INT PRIMARY KEY,  date DATE NOT NULL);
CREATE TABLE orderlineitem (  id INT PRIMARY KEY,  orderid INT REFERENCES "order"(id),  seq INT NOT NULL,  CONSTRAINT
con1UNIQUE (orderid, seq),  productid INT NOT NULL,  quantity INT NOT NULL,  price NUMERIC(18,4) NOT NULL);
 

how do we enforce things like, for example: a) each order must contain
at least 1 line item; or b) each order must have a non-zero total value;
without making INSERT painful? I'm new to triggers, so I'm thinking of
something like a "before COMMIT" trigger but there's no such thing
apparently.

Regards,
dave