SQL查询语句（二）—— 多表查询

一、列名/表名更改

列名更改

AS关键字是可选的，因此您可以在语句中将其省略

SELECT 
   [column_1 | expression] AS descriptive_name
FROM table_name;	

选择员工的名字和姓氏，使用CONCAT_WS()函数将名字和姓氏连接为全名

SELECT
   CONCAT_WS(', ', lastName, firstname) AS `Full name`
FROM
   employees;

表名更改

table_name AS table_alias

将employees表别名更改为e：

SELECT * FROM employees e;

为表分配别名后，您可以使用以下语法引用表列：

table_alias.column_name

SELECT 
    e.firstName, 
    e.lastName
FROM
    employees e
ORDER BY e.firstName;

SELECT
	customerName,
	COUNT(o.orderNumber) total
FROM
	customers c
INNER JOIN orders o ON c.customerNumber = o.customerNumber
GROUP BY
	customerName
ORDER BY
	total DESC;

二、INNER JOIN 内连接,等值连接

SELECT
    select_list
FROM t1
INNER JOIN t2 ON join_condition1
INNER JOIN t3 ON join_condition2
...;

示例：

查询 产品的号码、产品名、产品描述

SELECT 
    productCode, 
    productName, 
    textDescription
FROM
    products t1
INNER JOIN productlines t2 
    ON t1.productline = t2.productline;

由于两个表的联接列都具有相同的名称 productline，因此可以使用以下USING

SELECT 
    productCode, 
    productName, 
    textDescription
FROM
    products
INNER JOIN productlines USING (productline);

示例2：

从orders和orderdetails表中返回来自和表的订单号，订单状态和总销售额：INNER JOIN``GROUP BY

SELECT 
    t1.orderNumber,
    t1.status,
    SUM(quantityOrdered * priceEach) total
FROM
    orders t1
INNER JOIN orderdetails t2 
    ON t1.orderNumber = t2.orderNumber
GROUP BY orderNumber;

INNER JOIN–连接三个表示例：

SELECT 
    orderNumber,
    orderDate,
    orderLineNumber,
    productName,
    quantityOrdered,
    priceEach
FROM
    orders
INNER JOIN
    orderdetails USING (orderNumber)
INNER JOIN
    products USING (productCode)
ORDER BY 
    orderNumber, 
    orderLineNumber;

INNER JOIN`使用其他运算符

SELECT 
    orderNumber, 
    productName, 
    msrp, 
    priceEach
FROM
    products p
INNER JOIN orderdetails o 
   ON p.productcode = o.productcode
      AND p.msrp > o.priceEach
WHERE
    p.productcode = 'S10_1678';

三、LEFT JOIN 左连接

LEFT JOIN返回左表中的所有行，而不管左表中的行是否与右表中的行匹配。如果没有匹配项，则右表中该行的列将包含NULL

SELECT 
    select_list
FROM
    t1
LEFT JOIN t2 ON 
    join_condition;

SELECT
    c.customerNumber,
    customerName,
    orderNumber,
    status
FROM
    customers c
LEFT JOIN orders o 
    ON c.customerNumber = o.customerNumber;

LEFT JOIN 查找不匹配的行

SELECT 
    c.customerNumber, 
    c.customerName, 
    o.orderNumber, 
    o.status
FROM
    customers c
LEFT JOIN orders o 
    ON c.customerNumber = o.customerNumber
WHERE
    orderNumber IS NULL;

对INNER JOIN子句，该子句中on的使用 可以等效WHERE 但在LEFT JOIN不等效

SELECT 
    o.orderNumber, 
    customerNumber, 
    productCode
FROM
    orders o
LEFT JOIN orderDetails 
    USING (orderNumber)
WHERE
    orderNumber = 10123;

SELECT 
    o.orderNumber, 
    customerNumber, 
    productCode
FROM
    orders o
LEFT JOIN orderDetails d 
    ON o.orderNumber = d.orderNumber AND 
       o.orderNumber = 10123;

-Condition-in-WHERE-clause.png)

四、RIGHT JOIN 右链接

RIGHT JOIN返回右表中的所有行，而不管右表中的行是否与左表中的行匹配。如果没有匹配项，则右左表中该行的列将包含NULL

五、CROSS JOIN 交叉连接

从联接表中返回行的笛卡尔积

SELECT * FROM t1
CROSS JOIN t2;

获取所有商店和产品的组合

SELECT 
    store_name, product_name
FROM
    stores AS a
        CROSS JOIN
    products AS b;

https://www.mysqltutorial.org/mysql-cross-join/

六、SELF JOIN 自连接

SELECT 
    CONCAT(m.lastName, ', ', m.firstName) AS Manager,
    CONCAT(e.lastName, ', ', e.firstName) AS 'Direct report'
FROM
    employees e
INNER JOIN employees m ON 
    m.employeeNumber = e.reportsTo
ORDER BY 
    Manager;

SELECT 
    IFNULL(CONCAT(m.lastname, ', ', m.firstname),
            'Top Manager') AS 'Manager',
    CONCAT(e.lastname, ', ', e.firstname) AS 'Direct report'
FROM
    employees e
LEFT JOIN employees m ON 
    m.employeeNumber = e.reportsto
ORDER BY 
    manager DESC;

显示位于同一城市的客户列表

SELECT 
    c1.city, 
    c1.customerName, 
    c2.customerName
FROM
    customers c1
INNER JOIN customers c2 ON 
    c1.city = c2.city
    AND c1.customername > c2.customerName
ORDER BY 
    c1.city;

• c1.city = c2.city 确保两个客户都在同一个城市。
• c.customerName > c2.customerName 确保不包括同一位客户。

参考资料

basic-mysql-tutorial