MySQL多个内部联接？

Question

I have three tables...

create table customers (
customerID integer unsigned not null auto_increment,
customername varchar(45) not null,
address varchar(45) not null,
city varchar(45) not null,
state varchar(2) not null,
zip mediumint(5) zerofill not null,
primary key (customerID)
);

create table products (
prodID integer unsigned not null auto_increment,
prodname varchar(45) not null,
prodcat integer unsigned,
proddesc varchar(75) not null,
price float (8,2),
qoh integer not null,
prodiconurl varchar(45),
primary key (prodID),
constraint FK_CAT foreign key FK_CAT (prodcat)
    references categories (catID)
    on delete restrict
    on update restrict
);

create table SHOPPING_CART (
CustomerID INTEGER UNSIGNED NOT NULL,
ProductID INTEGER UNSIGNED NOT NULL,
ProdQnty INTEGER UNSIGNED NOT NULL,
Primary Key (CustomerID, ProductID),
CONSTRAINT Fk_Cust Foreign Key Fk_Cust (CustomerID)
    References Customers (CustID)
    On Delete restrict
    On Update restrict, 
CONSTRAINT Fk_Prod Foreign Key Fk_Prod (ProductID)
    References Products (ProdID)
    On Delete restrict
    On Update restrict
);

The tables are fine. what I'm having problems with is my SELECT statement for SHOPPING_CART. If I insert the following code.

insert into SHOPPING_CART
Values (
1, 4, 2
);
insert into SHOPPING_CART
values (
1, 9, 1
);

My inner join does not work for looking up the CustID, returning the FirstName and LastName fields from the Customers tables and returning the Product name from the Products table, based on values in the shopping cart table.

Here's my ultimate goal.

SELECT 
Customers.FirstName,
Customers.LastName,
Products.Name,
SHOPPING_CART.ProdQnty
FROM SHOPPING_CART
inner join Customers on SHOPPING_CART.CustomerID = Customers.CustID
where Customers.CustID = SHOPPING_CART.CustomerID
inner join Products on SHOPPING_CART.ProductID = Products.ProdID
where Products.ProdID = SHOPPING_CART.ProductID;

I get a statement error when the second inner join is part of the statement, also the statement has a red x on the second inner join.

Answer 1

Only 1 where statement is required. The second where statement would go underneath an "AND" statement. Note: The code is pre ANSI-92. Explanation of how to use ANSI-92 follows code example.

SELECT 
Customers.FirstName,
Customers.LastName,
Products.Name,
SHOPPING_CART.ProdQnty
FROM SHOPPING_CART
WHERE Customers.CustID = SHOPPING_CART.CustomerID
AND Products.ProdID = SHOPPING_CART.ProductID;

History:

Prior to ANSI 92, joins within the where statements were common. After the standardization, the joins moved from the where statement to the from statement. Where statements are used to provide additional filtering capability eg where status = 'open'. The following excerpt explains ANSI-92 join syntax.

The ANSI Join Syntax Before the ANSI SQL-92 standard introduced the new join syntax, relations (tables, views, etc.) were named in the FROM clause, separated by commas. Join conditions were specified in the WHERE clause:

=> SELECT * FROM T1, T2 WHERE T1.id = T2.id; The ANSI SQL-92 standard provided more specific join syntax, with join conditions named in the ON clause:

=> SELECT * FROM T1 [ INNER | LEFT OUTER | RIGHT OUTER | FULL OUTER | NATURAL | CROSS ] JOIN T2 ON T1.id = T2.id See SQL-99 ANSI syntax at BNF Grammar for SQL-99 for additional details.

Advantages:

SQL-92 outer join syntax is portable across databases; the older syntax was not consistent between databases. SQL-92 syntax provides greater control over whether predicates are to be evaluated during or after outer joins. This was also not consistent between databases when using the older syntax. See "Join Conditions vs. Filter Conditions" below. SQL-92 syntax eliminates ambiguity in the order of evaluating the joins, in cases where more than two tables are joined with outer joins. Union joins can be expressed using the SQL-92 syntax, but not in the older syntax.

Updated Query incorporating ANSI-92:

SELECT 
Customers.FirstName,
Customers.LastName,
Products.Name,
SHOPPING_CART.ProdQnty
FROM SHOPPING_CART
inner join Customers on SHOPPING_CART.CustomerID = Customers.CustID
inner join Products on SHOPPING_CART.ProductID = Products.ProdID

Answer 2

If you use JOIN then you not need where.

Your request will be

SELECT 
  Customers.FirstName,
  Customers.LastName,
  Products.Name,
  SHOPPING_CART.ProdQnty
FROM SHOPPING_CART
INNER JOIN Customers ON SHOPPING_CART.CustomerID = Customers.CustomerID
INNER JOIN Products on SHOPPING_CART.ProductID = Products.ProdID

And I found one error in on clause in your code. You used CustID for table Customers. But in your create table this fields is CustomerID. Don't know where is mistake - just wrote request for your structure.