foreign key references to a table problem here

create database Exer4

use Exer4

create table customer ( 
    cus_code int,
    constraint PK_customer primary key (cus_code),
    cus_Lname varchar(20),
    cus_Fname varchar (30),
    cus_intial varchar (2),
    cus_areacode int,
    cus_phone int,
    cus_balance float
     )

create table charter (
    char_trip int,
    constraint PK_charter primary key (char_trip),
    char_date date,
    ac_number varchar(5),
    foreign key(ac_number) references aircraft,
    char_destination varchar(5),
    char_distance float,
    char_hours_flown float,
    char_hours_wait float,
    char_fuel_gallons float,
    cus_code int,
    foreign key(cus_code) references customer
    )


    create table aircraft(

    ac_number varchar(5),
    constraint PK_aircraft primary key(ac_number),
    mod_code varchar(10),
    foreign key(mod_code) references model,
    ac_itaf varchar(10),
    ac_tiel varchar(10),
    ac_开发者_如何转开发tier varchar(10),
    )

    create table model(
    mod_code int,
    constraint PK_model primary key(mod_code),
    mod_manufacturer varchar(10),
    mod_name varchar(10),
    mod_seats int,
    mod_chg_mile int,
    )

create table crew (
    char_trip int,
    emp_num int,
    constraint PK_crew primary key (char_trip,emp_num),
    foreign key (char_trip) references charter,
    foreign key(emp_num) references employee,
    crew_job varchar(10)
    )


create table rating (
    rtg_code varchar(5),
    rtg_name varchar (30),
    constraint PK_rating primary key (rtg_code)
    )

create table employee (
    emp_num int,

    constraint PK_employee primary key (emp_num),
    emp_title varchar (4),
    emp_lname varchar (20),
    emp_fname varchar (30),
    emp_initial varchar (2),
    emp_dob date,
    emp_hire_date date, 
    )



create table pilot (
    emp_num int,
    pl_license varchar (3),
    pl_ratings varchar (30),
    pl_med_type int,
    pl_med_date date,
    pl_pt135_date date,
    constraint PK_pilot primary key (emp_num)
    )

why is that when i reference to "aircraft" it said invalid table?? what is wrong with my code??

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You have to specify which field to reference:

foreign key (mod_code) references model (mod_code),

The foreign key constraint consists of 2 parts:

ALTER TABLE aircraft
ADD CONSTRAINT fk_aircraft_model
FOREIGN KEY (mod_code)  -- here you specify the field(s) to reference from in the aircraft table
REFERENCES model (mod_code)  -- here you specify the field(s) to reference to in the model table

---

You are creating the tables in the wrong order. charter references aircraft but create table aircraft doesn't appear until later.