SQL联查案例

SQL 联查（Join）是将来自两个或多个表的数据结合在一起的操作。以下是一些常用的 SQL 联查示例，包括内联接、左联接、右联接和全外联接。

示例数据库表和数据

1. Customers (客户表)

CustomerID	Name	City
1	Alice	New York
2	Bob	Los Angeles
3	Charlie	Chicago
4	David	San Francisco
5	Eve	Miami

2. Orders (订单表)

OrderID	CustomerID	OrderDate	Amount
101	1	2025-01-15	150.00
102	1	2025-02-20	200.00
103	2	2025-01-25	300.00
104	3	2025-03-05	450.00
105	4	2025-04-10	500.00

查询和输出效果

1. 内联接 (INNER JOIN)

查询所有下过订单的客户及其订单信息：

SELECT 
    Customers.CustomerID,
    Customers.Name,
    Orders.OrderID,
    Orders.OrderDate,
    Orders.Amount
FROM 
    Customers
INNER JOIN 
    Orders ON Customers.CustomerID = Orders.CustomerID;

输出结果：

CustomerID	Name	OrderID	OrderDate	Amount
1	Alice	101	2025-01-15	150.00
1	Alice	102	2025-02-20	200.00
2	Bob	103	2025-01-25	300.00
3	Charlie	104	2025-03-05	450.00
4	David	105	2025-04-10	500.00

2. 左联接 (LEFT JOIN)

查询所有客户及其订单信息，包括没有下过订单的客户：

SELECT 
    Customers.CustomerID,
    Customers.Name,
    Orders.OrderID,
    Orders.OrderDate,
    Orders.Amount
FROM 
    Customers
LEFT JOIN 
    Orders ON Customers.CustomerID = Orders.CustomerID;

输出结果：

CustomerID	Name	OrderID	OrderDate	Amount
1	Alice	101	2025-01-15	150.00
1	Alice	102	2025-02-20	200.00
2	Bob	103	2025-01-25	300.00
3	Charlie	104	2025-03-05	450.00
4	David	105	2025-04-10	500.00
5	Eve	NULL	NULL	NULL

3. 右联接 (RIGHT JOIN)

查询所有订单及其客户信息，包括没有客户信息的订单：

SELECT 
    Customers.CustomerID,
    Customers.Name,
    Orders.OrderID,
    Orders.OrderDate,
    Orders.Amount
FROM 
    Customers
RIGHT JOIN 
    Orders ON Customers.CustomerID = Orders.CustomerID;

输出结果：

CustomerID	Name	OrderID	OrderDate	Amount
1	Alice	101	2025-01-15	150.00
1	Alice	102	2025-02-20	200.00
2	Bob	103	2025-01-25	300.00
3	Charlie	104	2025-03-05	450.00
4	David	105	2025-04-10	500.00

4. 全外联接 (FULL OUTER JOIN)

查询所有客户与所有订单的信息，包括没有匹配的客户和订单：

SELECT 
    Customers.CustomerID,
    Customers.Name,
    Orders.OrderID,
    Orders.OrderDate,
    Orders.Amount
FROM 
    Customers
FULL OUTER JOIN 
    Orders ON Customers.CustomerID = Orders.CustomerID;

输出结果：

CustomerID	Name	OrderID	OrderDate	Amount
1	Alice	101	2025-01-15	150.00
1	Alice	102	2025-02-20	200.00
2	Bob	103	2025-01-25	300.00
3	Charlie	104	2025-03-05	450.00
4	David	105	2025-04-10	500.00
5	Eve	NULL	NULL	NULL