SQL簡明例項教程

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

例如，列出所有人的訂單資訊

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

任何right join都可以換成left join的寫法，反之亦然。

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;

例如，如何查詢各個國家的所有客戶和**商？

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;

如果需要從兩個表中查詢指定的字段資訊，可以使用join;

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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