Keys

Primary keys

-- Primary key constraint
--
create table client (
    client_id      integer    primary key,
    ...
);

-- Composite primary key constraint
--
create table order (
    customer_id      integer,
    order_id      integer,
    ...
    constraint order_key primary key
        (customer_id, order_id)
);

-- "not null", "unique" contraints
--
create table client (
    client_id     integer    not null,
    ...
    unique (client_id)
);

Foreign keys

create table orders (
    client_id        integer,
    salesperson_id   integer,
    test_ordered_id  integer,
    ...
    constraint client_fk foreign key (client_id)
        references client (client_id)
            on delete cascade,
    ...
    constraint salesperson_fk foreign key (salesperson_id)
        references employee (employee_id)
            on delete set null,
    ...
    constraint test_ordered_fk foreign key (test_ordered_id)
        references tests (test_id)
);