了解一个表的主键如何也可以成为外键

Question

I've been given the code below:

---

-- Create Customers table

CREATE TABLE Customers
(
cust_id      char(10)  NOT NULL ,
cust_name    char(50)  NOT NULL ,
cust_address char(50)  NULL ,
cust_city    char(50)  NULL ,
cust_state   char(5)   NULL ,
cust_zip     char(10)  NULL ,
cust_country char(50)  NULL ,
cust_contact char(50)  NULL ,
cust_email   char(255) NULL 
);

---

-- Create OrderItems table

CREATE TABLE OrderItems
(
order_num  int          NOT NULL ,
order_item int          NOT NULL ,
prod_id    char(10)     NOT NULL ,
quantity   int          NOT NULL ,
item_price decimal(8,2) NOT NULL 
);

---

-- Create Orders table

CREATE TABLE Orders
(
order_num  int      NOT NULL ,
order_date datetime NOT NULL ,
cust_id    char(10) NOT NULL 
);

---

-- Create Products table

CREATE TABLE Products
(
prod_id    char(10)      NOT NULL ,
vend_id    char(10)      NOT NULL ,
prod_name  char(255)     NOT NULL ,
prod_price decimal(8,2)  NOT NULL ,
prod_desc  varchar(1000) NULL 
);

---

-- Create Vendors table

CREATE TABLE Vendors
(
vend_id      char(10) NOT NULL ,
vend_name    char(50) NOT NULL ,
vend_address char(50) NULL ,
vend_city    char(50) NULL ,
vend_state   char(5)  NULL ,
vend_zip     char(10) NULL ,
vend_country char(50) NULL 
);

---

-- Define primary keys

ALTER TABLE Customers WITH NOCHECK ADD CONSTRAINT PK_Customers PRIMARY KEY CLUSTERED (cust_id);
ALTER TABLE OrderItems WITH NOCHECK ADD CONSTRAINT PK_OrderItems PRIMARY KEY CLUSTERED     (order_num, order_item);
ALTER TABLE Orders WITH NOCHECK ADD CONSTRAINT PK_Orders PRIMARY KEY CLUSTERED (order_num);
ALTER TABLE Products WITH NOCHECK ADD CONSTRAINT PK_Products PRIMARY KEY CLUSTERED (prod_id);
ALTER TABLE Vendors WITH NOCHECK ADD CONSTRAINT PK_Vendors PRIMARY KEY CLUSTERED (vend_id);

---

-- Define foreign keys

 ALTER TABLE OrderItems ADD
 CONSTRAINT FK_OrderItems_Orders FOREIGN KEY (order_num) REFERENCES Orders (order_num),
 CONSTRAINT FK_OrderItems_Products FOREIGN KEY (prod_id) REFERENCES Products (prod_id);
 ALTER TABLE Orders ADD 
 CONSTRAINT FK_Orders_Customers FOREIGN KEY (cust_id) REFERENCES Customers (cust_id);
 ALTER TABLE Products ADD
 CONSTRAINT FK_Products_Vendors FOREIGN KEY (vend_id) REFERENCES Vendors (vend_id);

I have a couple of questions:

1. What's the purpose of the Composite Key in table Orders ?
2. Since order_num is a Primary Key as well as Foreign Key, I can't understand how it can be Clustered in one table and Non-Clustered in another.
3. Also, I don't understand the purpose of WITH NOCHECK when defining the Primary Keys.

Thanks

Answer 1

1. What does order_item mean? From the look of things, you can remove it from the PK.

2. An FK basically says "the value here must be one of the values in that other table". Think of a dropdown box: the list of available choices is provided that the column in the REFERENCES ... (...) clause. CLUSTERED has no impact on it. You can make an FK references non-primary key column too.

3. When you create the primary key, SQL Server checks the table to ensure that the PK column contains no duplicate value. WITH NOCHECK disables check on existing data, but will check for any new data coming into the table. Its usage is not recommended .