CREATE TABLE office(
   office_id    INTEGER  NOT NULL PRIMARY KEY 
  ,office_name VARCHAR(15) NOT NULL
);
CREATE TABLE category(
   category_id    INTEGER  NOT NULL PRIMARY KEY 
  ,category_name VARCHAR(15) NOT NULL
);
CREATE TABLE employee(
   employee_id    INTEGER  NOT NULL
  ,Firstname text NOT NULL
  ,birthdate DATE  NOT NULL
  ,Register  text NOT NULL
  ,Lastname  text NOT NULL
  ,Gender    text NOT NULL
  ,category_id INTEGER  NOT NULL 
  ,office_id INTEGER  NOT NULL 
  ,user_name text NOT NULL
  ,pass_word text NOT NULL,
    foreign key(category_id) references category(category_id),
    foreign key(office_id) references office(office_id),
    primary key(employee_id,category_id,office_id),
    UNIQUE(Firstname)
);
After this i want to create this table but it gives me "there is no unique constraint matching given keys for referenced table "employees" this mistake i don't know how to fix this?
CREATE TABLE employee_phone(
   employee_id INTEGER  NOT NULL
  ,phone INTEGER NOT NULL,
    foreign key(employee_id) references employees(employee_id),
    primary key(employee_id)
);
