SQL简明实例教程

创建客户customer表

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`)
);

创建供应商Supplier表

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`)
);

创建订单表orders

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`)
);

列出所有订单详情（OrderNumber, TotalAmount, FirstName, LastName, City, Country）

创建订单项表 orderitem

CREATE TABLE `orderitem` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `OrderId` int(11) NOT NULL,
  `ProductId` int(11) NOT NULL,
  `UnitPrice` float NOT NULL,
  `Quantity` int(11) NOT NULL,
  PRIMARY KEY (`id`)
);

创建Product表

CREATE TABLE `product2` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `ProductName` char(100) NOT NULL,
  `SupplierId` int(11) NOT NULL,
  `UnitPrice` float NOT NULL,
  `Package` int(11) NOT NULL,
  `IsDiscontinued` tinyint(4) NOT NULL COMMENT '是否下架',
  PRIMARY KEY (`id`)
);

从orders、orderitem、Product三个表中查询OrderNumber、ProductName、UnitPrice、Quantity

SELECT
    O.OrderNumber as 订单号,
    O.OrderDate AS 订购日期,
    P.ProductName AS 物品名称,
    I.Quantity as 购买数量,
    I.UnitPrice as 单价
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

列出第一个(左边的)表的所有记录对应的第二个(右边的)表信息

例如，列出所有人的订单信息

SELECT
    c.FirstName,
    c.LastName,
    c.City,
    c.Country,
    o.OrderNumber,
    o.TotalAmount
FROM
    Customer C  -- Customer表中的每一个人
LEFT JOIN Orders O   -- Orders就是每一个人对应的订单信息

ON O.CustomerId = C.Id  -- Customer 与  Orders的 联系
ORDER BY   -- 排序
    TotalAmount

LEFT JOIN和RIGHT JOIN的区别和联系

任何RIGHT JOIN都可以换成LEFT JOIN的写法，反之亦然。
例如，下面两种写法的查询结果是等价的：

-- 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;

FULL JOIN

例如，如何查询各个国家的所有客户和供应商？
分析：
1. 查询各个国家，需要按国家排序
2. 查询客户和供应商，需要从Customer表和Supplier表中查询；
3. 查询所有客户和所有供应商，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;

总结：

如果需要从两个表中查询指定的字段信息，可以使用JOIN;
如果同时查询所有客户对应的供应商信息，可以用FROM Customer LEFT JOIN Supplier;
如果同时查询所有供应商对应的客户信息，可以用FROM Customer RIGHT JOIN Supplier;
如果同时查询所有客户和所有供应商的信息，可以用FROM Customer FULL JOIN SupplierSupplier;
有些数据库不支持FULL JOIN，例如MySQL，不过在MySQL中可以使用LEFT JOIN + UNION + RIGHT JOIN 实现类似FULL JOIN的功能，例如上述例子如果在 MySQL中可以这样写：

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;