create
table
`customer` (
`id`
int(11) not
null auto_increment,
`lastname`
char(50) default
null,
`firstname`
char(50) default
null,
`address`
char(50) default
null,
`city`
char(50) default
null,
`county`
char(50) default
null,
`phone`
char(20) default
null,
primary
key (`id`)
);
create
table
`supplier` (
`id`
int(11) not
null auto_increment,
`companyname`
char(50) default
null,
`contactname`
char(50) default
null,
`city`
char(50) default
null,
`county`
char(50) default
null,
`phone`
char(20) default
null,
`fax`
char(20) default
null,
primary
key (`id`)
);
create
table
`orders` (
`id`
int(11) not
null auto_increment,
`orderdate` datetime default
null,
`ordernumber`
char(30) not
null,
`customerid`
int(11) not
null,
`totalamount`
int(11) not
null,
primary
key (`id`)
);
create
table
`orderitem` (
`id`
int(11) not
null auto_increment,
`orderid`
int(11) not
null,
`productid`
int(11) not
null,
`unitprice`
float
notnull,
`quantity`
int(11) not
null,
primary
key (`id`)
);
create
table
`product2` (
`id`
int(11) not
null auto_increment,
`productname`
char(100) not
null,
`supplierid`
int(11) not
null,
`unitprice`
float
notnull,
`package`
int(11) not
null,
`isdiscontinued` tinyint(4) not
null comment '是否下架',
primary
key (`id`)
);
select
o.ordernumber as 訂單號,
o.orderdate as 訂購日期,
p.productname as 物品名稱,
i.quantity as 購買數量,
i.unitprice as 單價
from
orders o
join orderitem i on o.id = i.orderid -- join 即 inner
join
join product p on p.id = i.productid
order
by o.ordernumber
select
c.firstname,
c.lastname,
c.city,
c.country,
o.ordernumber,
o.totalamount
from
customer c -- customer表中的每乙個人
left
join orders o -- orders就是每乙個人對應的訂單資訊
on o.customerid = c.id -- customer 與 orders的 聯絡
order
by -- 排序
totalamount
例如，下面兩種寫法的查詢結果是等價的：
-- left join
select
totalamount,
firstname,
lastname,
city,
country
from
customer c
left
join orders o on o.customerid = c.id
where
totalamount is
null;
-- right join
select
totalamount,
firstname,
lastname,
city,
country
from
orders o
right
join customer c on o.customerid = c.id
where
totalamount is
null;
分析：
1. 查詢各個國家，需要按國家排序
2. 查詢客戶和**商，需要從customer表和supplier表中查詢；
3. 查詢所有客戶和所有**商，需要使用full join;
select c.firstname, c.lastname, c.country as customercountry, s.country as suppliercountry, s.companyname from customer c full
join supplier s on c.country = s.country order
by c.country, s.country;
如果同時查詢所有客戶對應的**商資訊，可以用from customer left join supplier;
如果同時查詢所有**商對應的客戶資訊，可以用from customer right join supplier;
如果同時查詢所有客戶和所有**商的資訊，可以用from customer full join suppliersupplier;
有些資料庫不支援full join，例如mysql，不過在mysql中可以使用left join + union + right join 實現類似full join的功能，例如上述例子如果在 mysql中可以這樣寫：
select c.firstname, c.lastname, c.country as customercountry, s.country as suppliercountry, s.companyname from customer c left
join supplier s on c.country = s.country
union
select c.firstname, c.lastname, c.country as customercountry, s.country as suppliercountry, s.companyname from customer c right
join supplier s on c.country = s.country order
by suppliercountry,customercountry;
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