多表查询

多表查询，也称为关联查询，指两个或更多个表一起完成查询操作。

前提条件：这些一起查询的表之间是有关系的（一对一、一对多），它们之间一定是有关联字段，这个关联字段可能建立了外键，也可能没有建立外键。比如：员工表和部门表，这两个表依靠“部门编号”进行关联。

一、一个案例引发的多表连接

1. 案例说明

从多个表中获取数据：

#案例：查询员工的姓名及其部门名称
SELECT last_name, department_name FROM employees, departments;
查询结果：
+-----------+----------------------+
| last_name | department_name      |
+-----------+----------------------+
| King      | Administration       |
| King      | Marketing            |
| King      | Purchasing           |
| King      | Human Resources      |
| King      | Shipping             |
| King      | IT                   |
| King      | Public Relations     |
| King      | Sales                |
| King      | Executive            |
| King      | Finance              |
| King      | Accounting           |
| ....      | ........             |
| Gietz     | IT Support           |
| Gietz     | NOC                  |
| Gietz     | IT Helpdesk          |
| Gietz     | Government Sales     |
| Gietz     | Retail Sales         |
| Gietz     | Recruiting           |
| Gietz     | Payroll              |
+-----------+----------------------+
2889 rows in set (0.01 sec)

分析错误情况：

SELECT COUNT(employee_id) FROM employees;
#输出107行

SELECT COUNT(department_id)FROM departments;
#输出27行

SELECT 107*27 FROM dual;
#输出2889行

我们把上述多表查询中出现的问题称为：笛卡尔积的错误。

2. 笛卡尔积（或交叉连接）的理解

笛卡尔乘积是一个数学运算。假设我有两个集合 X 和 Y，那么 X 和 Y 的笛卡尔积就是 X 和 Y 的所有可能组合，也就是第一个对象来自于 X，第二个对象来自于 Y 的所有可能。组合的个数即为两个集合中元素个数的乘积数。

SQL92中，笛卡尔积也称为交叉连接，英文是 CROSS JOIN。在 SQL99 中也是使用 CROSS JOIN表示交叉连接。它的作用就是可以把任意表进行连接，即使这两张表不相关。在MySQL中如下情况会出现笛卡尔积：

#查询员工姓名和所在部门名称
SELECT last_name,department_name FROM employees,departments;
SELECT last_name,department_name FROM employees CROSS JOIN departments;
SELECT last_name,department_name FROM employees INNER JOIN departments;
SELECT last_name,department_name FROM employees JOIN departments;

3. 案例分析与问题解决

笛卡尔积的错误会在下面条件下产生：
- 省略多个表的连接条件（或关联条件）
- 连接条件（或关联条件）无效
- 所有表中的所有行互相连接
为了避免笛卡尔积，可以在 WHERE 加入有效的连接条件。

加入连接条件后，查询语法：

SELECT	table1.column, table2.column
FROM	table1, table2
WHERE	table1.column1 = table2.column2;  #连接条件

在 WHERE子句中写入连接条件。

正确写法：

#案例：查询员工的姓名及其部门名称
SELECT last_name, department_name
FROM employees, departments
WHERE employees.department_id = departments.department_id;

在表中有相同列时，在列名之前加上表名前缀。

二、多表查询分类

1. 等值连接 vs 非等值连接

等值连接

SELECT employees.employee_id, employees.last_name, 
       employees.department_id, departments.department_id,
       departments.location_id
FROM   employees, departments
WHERE  employees.department_id = departments.department_id;

表的别名

使用别名可以简化查询。
列名前使用表名前缀可以提高查询效率。

SELECT e.employee_id, e.last_name, e.department_id,
       d.department_id, d.location_id
FROM   employees e , departments d
WHERE  e.department_id = d.department_id;

需要注意的是，如果我们使用了表的别名，在查询字段中、过滤条件中就只能使用别名进行代替，不能使用原有的表名，否则就会报错。

阿里开发规范：

【强制】对于数据库中表记录的查询和变更，只要涉及多个表，都需要在列名前加表的别名（或表名）进行限定。

说明：对多表进行查询记录、更新记录、删除记录时，如果对操作列没有限定表的别名（或表名），并且操作列在多个表中存在时，就会抛异常。

正例：select t1.name from table_first as t1 , table_second as t2 where t1.id=t2.id;

反例：在某业务中，由于多表关联查询语句没有加表的别名（或表名）的限制，正常运行两年后，最近在某个表中增加一个同名字段，在预发布环境做数据库变更后，线上查询语句出现出 1052 异常：Column 'name' in field list is ambiguous。

非等值连接

SELECT e.last_name, e.salary, j.grade_level
FROM   employees e, job_grades j
WHERE  e.salary BETWEEN j.lowest_sal AND j.highest_sal;

2. 自连接 vs 非自连接

当table1和table2本质上是同一张表，只是用取别名的方式虚拟成两张表以代表不同的意义。然后两个表再进行内连接，外连接等查询。
例如：查询employees表，返回“Xxx works for Xxx”

SELECT CONCAT(worker.last_name ,' works for ' 
       , manager.last_name)
FROM   employees worker, employees manager
WHERE  worker.manager_id = manager.employee_id ;

3. 内连接 vs 外连接

除了查询满足条件的记录以外，外连接还可以查询某一方不满足条件的记录。

内连接: 合并具有同一列的两个以上的表的行, 结果集中不包含一个表与另一个表不匹配的行
外连接: 两个表在连接过程中除了返回满足连接条件的行以外还返回左（或右）表中不满足条件的行 ，这种连接称为左（或右）外连接。没有匹配的行时, 结果表中相应的列为空(NULL)。
- 如果是左外连接，则连接条件中左边的表也称为主表，右边的表称为从表。
- 如果是右外连接，则连接条件中右边的表也称为主表，左边的表称为从表。

内连接(INNER JOIN)

创建 customers 表：

CREATE TABLE customers (
	id INT PRIMARY KEY,
    customer_id INT,
    customer_name VARCHAR(50)
);

插入数据到 customers 表：

INSERT INTO customers (id, customer_id, customer_name)
VALUES
    (1,0001,'John'),
    (2,0010, 'Jane'),
    (3,0011, 'Smith'),
    (4,0100, 'Michael'),
    (5,0101, NULL),
    (6,NULL, 'JACK'),
    (7,0111, 'Sarah');

创建 orders 表：

CREATE TABLE orders (
	id INT PRIMARY KEY,
    order_id INT,
    customer_id INT,
    order_date DATE,
    amount DECIMAL(10, 2)
);

插入数据到 orders 表：

INSERT INTO orders (id, order_id, customer_id, order_date, amount)
VALUES
    (1, 1,0001, '2023-01-01', 100.00),
    (2, 1,0010, '2023-02-01', 150.00),
    (3, 2,1001, '2023-03-01', 200.00),
    (4, 3,0011, NULL, 50.00),
    (5, 4,NULL, '2023-04-01', NULL),
    (6, NULL,NULL, NULL, NULL);

mysql> select * from customers;
+----+-------------+---------------+
| id | customer_id | customer_name |
+----+-------------+---------------+
|  1 |           1 | John          |
|  2 |          10 | Jane          |
|  3 |          11 | Smith         |
|  4 |         100 | Michael       |
|  5 |         101 | NULL          |
|  6 |        NULL | JACK          |
|  7 |         111 | Sarah         |
+----+-------------+---------------+
7 rows in set (0.00 sec)

mysql> select * from orders;
+----+----------+-------------+------------+--------+
| id | order_id | customer_id | order_date | amount |
+----+----------+-------------+------------+--------+
|  1 |        1 |           1 | 2023-01-01 | 100.00 |
|  2 |        1 |          10 | 2023-02-01 | 150.00 |
|  3 |        2 |        1001 | 2023-03-01 | 200.00 |
|  4 |        3 |          11 | NULL       |  50.00 |
|  5 |        4 |        NULL | 2023-04-01 |   NULL |
|  6 |     NULL |        NULL | NULL       |   NULL |
+----+----------+-------------+------------+--------+
6 rows in set (0.00 sec)

三、9种join方式

INNER JOIN

mysql> select * from customers;
+----+-------------+---------------+
| id | customer_id | customer_name |
+----+-------------+---------------+
|  1 |           1 | John          |
|  2 |          10 | Jane          |
|  3 |          11 | Smith         |
|  4 |         100 | Michael       |
|  5 |         101 | NULL          |
|  6 |        NULL | JACK          |
|  7 |         111 | Sarah         |
+----+-------------+---------------+
7 rows in set (0.00 sec)

mysql> select * from orders;
+----+----------+-------------+------------+--------+
| id | order_id | customer_id | order_date | amount |
+----+----------+-------------+------------+--------+
|  1 |        1 |           1 | 2023-01-01 | 100.00 |
|  2 |        1 |          10 | 2023-02-01 | 150.00 |
|  3 |        2 |        1001 | 2023-03-01 | 200.00 |
|  4 |        3 |          11 | NULL       |  50.00 |
|  5 |        4 |        NULL | 2023-04-01 |   NULL |
|  6 |     NULL |        NULL | NULL       |   NULL |
+----+----------+-------------+------------+--------+
6 rows in set (0.00 sec)

mysql> SELECT  o.order_id, o.customer_id, o.order_date, o.amount, c.customer_name
    -> FROM orders o
    -> INNER JOIN customers c
    -> ON o.customer_id = c.customer_id;
+----------+-------------+------------+--------+---------------+
| order_id | customer_id | order_date | amount | customer_name |
+----------+-------------+------------+--------+---------------+
|        1 |           1 | 2023-01-01 | 100.00 | John          |
|        1 |          10 | 2023-02-01 | 150.00 | Jane          |
|        3 |          11 | NULL       |  50.00 | Smith         |
+----------+-------------+------------+--------+---------------+
3 rows in set (0.00 sec)

LEFT JOIN

左外连接，则连接条件中左边的表也称为主表，右边的表称为从表。

mysql> SELECT * FROM orders;
+----+----------+-------------+------------+--------+
| id | order_id | customer_id | order_date | amount |
+----+----------+-------------+------------+--------+
|  1 |        1 |           1 | 2023-01-01 | 100.00 |
|  2 |        1 |          10 | 2023-02-01 | 150.00 |
|  3 |        2 |        1001 | 2023-03-01 | 200.00 |
|  4 |        3 |          11 | NULL       |  50.00 |
|  5 |        4 |        NULL | 2023-04-01 |   NULL |
|  6 |     NULL |        NULL | NULL       |   NULL |
+----+----------+-------------+------------+--------+
6 rows in set (0.00 sec)

mysql> SELECT * FROM customers;
+----+-------------+---------------+
| id | customer_id | customer_name |
+----+-------------+---------------+
|  1 |           1 | John          |
|  2 |          10 | Jane          |
|  3 |          11 | Smith         |
|  4 |         100 | Michael       |
|  5 |         101 | NULL          |
|  6 |        NULL | JACK          |
|  7 |         111 | Sarah         |
+----+-------------+---------------+
7 rows in set (0.00 sec)

mysql> SELECT  o.order_id, o.customer_id, c.customer_id, o.order_date, o.amount, c.customer_name
    -> FROM orders o
    -> LEFT JOIN customers c
    -> ON o.customer_id = c.customer_id;
+----------+-------------+-------------+------------+--------+---------------+
| order_id | customer_id | customer_id | order_date | amount | customer_name |
+----------+-------------+-------------+------------+--------+---------------+
|        1 |           1 |           1 | 2023-01-01 | 100.00 | John          |
|        1 |          10 |          10 | 2023-02-01 | 150.00 | Jane          |
|        2 |        1001 |        NULL | 2023-03-01 | 200.00 | NULL          |
|        3 |          11 |          11 | NULL       |  50.00 | Smith         |
|        4 |        NULL |        NULL | 2023-04-01 |   NULL | NULL          |
|     NULL |        NULL |        NULL | NULL       |   NULL | NULL          |
+----------+-------------+-------------+------------+--------+---------------+
6 rows in set (0.00 sec)

mysql> SELECT  o.order_id, o.customer_id, c.customer_id, o.order_date, o.amount, c.customer_name
    -> FROM customers c
    -> LEFT JOIN orders o
    -> ON o.customer_id = c.customer_id;
+----------+-------------+-------------+------------+--------+---------------+
| order_id | customer_id | customer_id | order_date | amount | customer_name |
+----------+-------------+-------------+------------+--------+---------------+
|        1 |           1 |           1 | 2023-01-01 | 100.00 | John          |
|        1 |          10 |          10 | 2023-02-01 | 150.00 | Jane          |
|        3 |          11 |          11 | NULL       |  50.00 | Smith         |
|     NULL |        NULL |         100 | NULL       |   NULL | Michael       |
|     NULL |        NULL |         101 | NULL       |   NULL | NULL          |
|     NULL |        NULL |        NULL | NULL       |   NULL | JACK          |
|     NULL |        NULL |         111 | NULL       |   NULL | Sarah         |
+----------+-------------+-------------+------------+--------+---------------+
7 rows in set (0.00 sec)

RIGHT JOIN

右外连接，则连接条件中右边的表也称为主表，左边的表称为从表。

mysql> SELECT * FROM orders;
+----+----------+-------------+------------+--------+
| id | order_id | customer_id | order_date | amount |
+----+----------+-------------+------------+--------+
|  1 |        1 |           1 | 2023-01-01 | 100.00 |
|  2 |        1 |          10 | 2023-02-01 | 150.00 |
|  3 |        2 |        1001 | 2023-03-01 | 200.00 |
|  4 |        3 |          11 | NULL       |  50.00 |
|  5 |        4 |        NULL | 2023-04-01 |   NULL |
|  6 |     NULL |        NULL | NULL       |   NULL |
+----+----------+-------------+------------+--------+
6 rows in set (0.00 sec)

mysql> SELECT * FROM customers;
+----+-------------+---------------+
| id | customer_id | customer_name |
+----+-------------+---------------+
|  1 |           1 | John          |
|  2 |          10 | Jane          |
|  3 |          11 | Smith         |
|  4 |         100 | Michael       |
|  5 |         101 | NULL          |
|  6 |        NULL | JACK          |
|  7 |         111 | Sarah         |
+----+-------------+---------------+
7 rows in set (0.00 sec)

mysql> SELECT  o.order_id, o.customer_id, c.customer_id, o.order_date, o.amount, c.customer_name
    -> FROM orders o
    -> RIGHT JOIN customers c
    -> ON o.customer_id = c.customer_id
    -> WHERE c.customer_id IS NULL;
+----------+-------------+-------------+------------+--------+---------------+
| order_id | customer_id | customer_id | order_date | amount | customer_name |
+----------+-------------+-------------+------------+--------+---------------+
|     NULL |        NULL |        NULL | NULL       |   NULL | JACK          |
+----------+-------------+-------------+------------+--------+---------------+
1 row in set (0.00 sec)

mysql> SELECT  o.order_id, o.customer_id, c.customer_id, o.order_date, o.amount, c.customer_name
    -> FROM customers c
    -> RIGHT JOIN orders o
    -> ON o.customer_id = c.customer_id;
+----------+-------------+-------------+------------+--------+---------------+
| order_id | customer_id | customer_id | order_date | amount | customer_name |
+----------+-------------+-------------+------------+--------+---------------+
|        1 |           1 |           1 | 2023-01-01 | 100.00 | John          |
|        1 |          10 |          10 | 2023-02-01 | 150.00 | Jane          |
|        2 |        1001 |        NULL | 2023-03-01 | 200.00 | NULL          |
|        3 |          11 |          11 | NULL       |  50.00 | Smith         |
|        4 |        NULL |        NULL | 2023-04-01 |   NULL | NULL          |
|     NULL |        NULL |        NULL | NULL       |   NULL | NULL          |
+----------+-------------+-------------+------------+--------+---------------+
6 rows in set (0.00 sec)

LEFT JOIN WHERE IS NULL

mysql> SELECT * FROM orders;
+----+----------+-------------+------------+--------+
| id | order_id | customer_id | order_date | amount |
+----+----------+-------------+------------+--------+
|  1 |        1 |           1 | 2023-01-01 | 100.00 |
|  2 |        1 |          10 | 2023-02-01 | 150.00 |
|  3 |        2 |        1001 | 2023-03-01 | 200.00 |
|  4 |        3 |          11 | NULL       |  50.00 |
|  5 |        4 |        NULL | 2023-04-01 |   NULL |
|  6 |     NULL |        NULL | NULL       |   NULL |
+----+----------+-------------+------------+--------+
6 rows in set (0.00 sec)

mysql> SELECT * FROM customers;
+----+-------------+---------------+
| id | customer_id | customer_name |
+----+-------------+---------------+
|  1 |           1 | John          |
|  2 |          10 | Jane          |
|  3 |          11 | Smith         |
|  4 |         100 | Michael       |
|  5 |         101 | NULL          |
|  6 |        NULL | JACK          |
|  7 |         111 | Sarah         |
+----+-------------+---------------+
7 rows in set (0.00 sec)

mysql> SELECT  o.order_id, o.customer_id, c.customer_id, o.order_date, o.amount, c.customer_name
    -> FROM orders o
    -> LEFT JOIN customers c
    -> ON o.customer_id = c.customer_id;
+----------+-------------+-------------+------------+--------+---------------+
| order_id | customer_id | customer_id | order_date | amount | customer_name |
+----------+-------------+-------------+------------+--------+---------------+
|        1 |           1 |           1 | 2023-01-01 | 100.00 | John          |
|        1 |          10 |          10 | 2023-02-01 | 150.00 | Jane          |
|        2 |        1001 |        NULL | 2023-03-01 | 200.00 | NULL          |
|        3 |          11 |          11 | NULL       |  50.00 | Smith         |
|        4 |        NULL |        NULL | 2023-04-01 |   NULL | NULL          |
|     NULL |        NULL |        NULL | NULL       |   NULL | NULL          |
+----------+-------------+-------------+------------+--------+---------------+
6 rows in set (0.00 sec)

mysql> SELECT  o.order_id, o.customer_id, c.customer_id, o.order_date, o.amount, c.customer_name
    -> FROM orders o
    -> LEFT JOIN customers c
    -> ON o.customer_id = c.customer_id
    -> WHERE c.customer_id IS NULL;
+----------+-------------+-------------+------------+--------+---------------+
| order_id | customer_id | customer_id | order_date | amount | customer_name |
+----------+-------------+-------------+------------+--------+---------------+
|        2 |        1001 |        NULL | 2023-03-01 | 200.00 | NULL          |
|        4 |        NULL |        NULL | 2023-04-01 |   NULL | NULL          |
|     NULL |        NULL |        NULL | NULL       |   NULL | NULL          |
+----------+-------------+-------------+------------+--------+---------------+
3 rows in set (0.00 sec)

RIGHT JOIN WHERE IS NULL

mysql> SELECT * FROM orders;
+----+----------+-------------+------------+--------+
| id | order_id | customer_id | order_date | amount |
+----+----------+-------------+------------+--------+
|  1 |        1 |           1 | 2023-01-01 | 100.00 |
|  2 |        1 |          10 | 2023-02-01 | 150.00 |
|  3 |        2 |        1001 | 2023-03-01 | 200.00 |
|  4 |        3 |          11 | NULL       |  50.00 |
|  5 |        4 |        NULL | 2023-04-01 |   NULL |
|  6 |     NULL |        NULL | NULL       |   NULL |
+----+----------+-------------+------------+--------+
6 rows in set (0.00 sec)

mysql> SELECT * FROM customers;
+----+-------------+---------------+
| id | customer_id | customer_name |
+----+-------------+---------------+
|  1 |           1 | John          |
|  2 |          10 | Jane          |
|  3 |          11 | Smith         |
|  4 |         100 | Michael       |
|  5 |         101 | NULL          |
|  6 |        NULL | JACK          |
|  7 |         111 | Sarah         |
+----+-------------+---------------+
7 rows in set (0.00 sec)

mysql> SELECT  o.order_id, o.customer_id, c.customer_id, o.order_date, o.amount, c.customer_name
    -> FROM orders o
    -> RIGHT JOIN customers c
    -> ON o.customer_id = c.customer_id;
+----------+-------------+-------------+------------+--------+---------------+
| order_id | customer_id | customer_id | order_date | amount | customer_name |
+----------+-------------+-------------+------------+--------+---------------+
|        1 |           1 |           1 | 2023-01-01 | 100.00 | John          |
|        1 |          10 |          10 | 2023-02-01 | 150.00 | Jane          |
|        3 |          11 |          11 | NULL       |  50.00 | Smith         |
|     NULL |        NULL |         100 | NULL       |   NULL | Michael       |
|     NULL |        NULL |         101 | NULL       |   NULL | NULL          |
|     NULL |        NULL |        NULL | NULL       |   NULL | JACK          |
|     NULL |        NULL |         111 | NULL       |   NULL | Sarah         |
+----------+-------------+-------------+------------+--------+---------------+
7 rows in set (0.00 sec)

mysql> SELECT  o.order_id, o.customer_id, c.customer_id, o.order_date, o.amount, c.customer_name
    -> FROM orders o
    -> RIGHT JOIN customers c
    -> ON o.customer_id = c.customer_id
    -> WHERE o.customer_id IS NULL;
+----------+-------------+-------------+------------+--------+---------------+
| order_id | customer_id | customer_id | order_date | amount | customer_name |
+----------+-------------+-------------+------------+--------+---------------+
|     NULL |        NULL |         100 | NULL       |   NULL | Michael       |
|     NULL |        NULL |         101 | NULL       |   NULL | NULL          |
|     NULL |        NULL |        NULL | NULL       |   NULL | JACK          |
|     NULL |        NULL |         111 | NULL       |   NULL | Sarah         |
+----------+-------------+-------------+------------+--------+---------------+
4 rows in set (0.00 sec)

四、UNION的使用

合并查询结果
利用UNION关键字，可以给出多条SELECT语句，并将它们的结果组合成单个结果集。合并时，两个表对应的列数和数据类型必须相同，并且相互对应。各个SELECT语句之间使用UNION或UNION ALL关键字分隔。

语法格式：

SELECT column,... FROM table1
UNION [ALL]
SELECT column,... FROM table2

UNION操作符

UNION 操作符返回两个查询的结果集的并集，去除重复记录。

UNION ALL操作符

UNION ALL操作符返回两个查询的结果集的并集。对于两个结果集的重复部分，不去重。

注意：执行UNION ALL语句时所需要的资源比UNION语句少。如果明确知道合并数据后的结果数据不存在重复数据，或者不需要去除重复的数据，则尽量使用UNION ALL语句，以提高数据查询的效率。

举例：查询部门编号>90或邮箱包含a的员工信息

#方式1
SELECT * FROM employees WHERE email LIKE '%a%' OR department_id>90;

#方式2
SELECT * FROM employees  WHERE email LIKE '%a%'
UNION
SELECT * FROM employees  WHERE department_id>90;

举例：查询中国用户中男性的信息以及美国用户中年男性的用户信息

SELECT id,cname FROM t_chinamale WHERE csex='男'
UNION ALL
SELECT id,tname FROM t_usmale WHERE tGender='male';

五、扩展：SQL99语法新特性

1.自然连接

SQL99 在 SQL92 的基础上提供了一些特殊语法，比如 NATURAL JOIN 用来表示自然连接。我们可以把自然连接理解为 SQL92 中的等值连接。它会帮你自动查询两张连接表中所有相同的字段，然后进行等值连接。

在SQL92标准中：

SELECT employee_id,last_name,department_name
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`
AND e.`manager_id` = d.`manager_id`;

在 SQL99 中你可以写成：

SELECT employee_id,last_name,department_name
FROM employees e NATURAL JOIN departments d;

2.USING连接

当我们进行连接的时候，SQL99还支持使用 USING 指定数据表里的同名字段进行等值连接。但是只能配合JOIN一起使用。比如：

SELECT employee_id,last_name,department_name
FROM employees e JOIN departments d
USING (department_id);

你能看出与自然连接 NATURAL JOIN 不同的是，USING 指定了具体的相同的字段名称，你需要在 USING 的括号 () 中填入要指定的同名字段。同时使用 JOIN...USING 可以简化 JOIN ON 的等值连接。它与下面的 SQL 查询结果是相同的：

SELECT employee_id,last_name,department_name
FROM employees e ,departments d
WHERE e.department_id = d.department_id;

多表连接查询

在增加一个顾客地址表

CREATE TABLE city (
	  id INT PRIMARY KEY,
    customer_name VARCHAR(50),
		location_city VARCHAR(50)
);

INSERT INTO city (id, customer_name, location_city)
VALUES
    (1,'John','四川'),
    (2, 'Jane','北京'),
    (3, 'Smith','上海'),
    (4, 'JACK','杭州'),
    (5, 'Sarah','深圳');

查询三张表

mysql> SELECT * FROM  orders;
+----+----------+-------------+------------+--------+
| id | order_id | customer_id | order_date | amount |
+----+----------+-------------+------------+--------+
|  1 |        1 |           1 | 2023-01-01 | 100.00 |
|  2 |        1 |          10 | 2023-02-01 | 150.00 |
|  3 |        2 |        1001 | 2023-03-01 | 200.00 |
|  4 |        3 |          11 | NULL       |  50.00 |
|  5 |        4 |        NULL | 2023-04-01 |   NULL |
|  6 |     NULL |        NULL | NULL       |   NULL |
+----+----------+-------------+------------+--------+
6 rows in set (0.00 sec)

mysql> SELECT * FROM  customers;
+----+-------------+---------------+
| id | customer_id | customer_name |
+----+-------------+---------------+
|  1 |           1 | John          |
|  2 |          10 | Jane          |
|  3 |          11 | Smith         |
|  4 |         100 | Michael       |
|  5 |         101 | NULL          |
|  6 |        NULL | JACK          |
|  7 |         111 | Sarah         |
+----+-------------+---------------+
7 rows in set (0.00 sec)

mysql> SELECT * FROM  city;
+----+---------------+---------------+
| id | customer_name | location_city |
+----+---------------+---------------+
|  1 | John          | 四川          |
|  2 | Jane          | 北京          |
|  3 | Smith         | 上海          |
|  4 | JACK          | 杭州          |
|  5 | Sarah         | 深圳          |
+----+---------------+---------------+
5 rows in set (0.00 sec)

多表连接查询

mysql> SELECT * FROM  city;
+----+---------------+---------------+
| id | customer_name | location_city |
+----+---------------+---------------+
|  1 | John          | 四川          |
|  2 | Jane          | 北京          |
|  3 | Smith         | 上海          |
|  4 | JACK          | 杭州          |
|  5 | Sarah         | 深圳          |
+----+---------------+---------------+
5 rows in set (0.00 sec)

mysql> SELECT * FROM  customers;
+----+-------------+---------------+
| id | customer_id | customer_name |
+----+-------------+---------------+
|  1 |           1 | John          |
|  2 |          10 | Jane          |
|  3 |          11 | Smith         |
|  4 |         100 | Michael       |
|  5 |         101 | NULL          |
|  6 |        NULL | JACK          |
|  7 |         111 | Sarah         |
+----+-------------+---------------+
7 rows in set (0.00 sec)

mysql> SELECT * FROM  orders;
+----+----------+-------------+------------+--------+
| id | order_id | customer_id | order_date | amount |
+----+----------+-------------+------------+--------+
|  1 |        1 |           1 | 2023-01-01 | 100.00 |
|  2 |        1 |          10 | 2023-02-01 | 150.00 |
|  3 |        2 |        1001 | 2023-03-01 | 200.00 |
|  4 |        3 |          11 | NULL       |  50.00 |
|  5 |        4 |        NULL | 2023-04-01 |   NULL |
|  6 |     NULL |        NULL | NULL       |   NULL |
+----+----------+-------------+------------+--------+
6 rows in set (0.00 sec)

mysql> SELECT   o.order_id, o.customer_id, o.order_date, o.amount, c.customer_id, c.customer_name, d.customer_name, d.location_city
    -> FROM orders o
    -> LEFT JOIN customers c
    -> ON o.customer_id = c.customer_id
    -> LEFT JOIN city d
    -> ON c.customer_name = d.customer_name;
+----------+-------------+------------+--------+-------------+---------------+---------------+---------------+
| order_id | customer_id | order_date | amount | customer_id | customer_name | customer_name | location_city |
+----------+-------------+------------+--------+-------------+---------------+---------------+---------------+
|        1 |           1 | 2023-01-01 | 100.00 |           1 | John          | John          | 四川          |
|        1 |          10 | 2023-02-01 | 150.00 |          10 | Jane          | Jane          | 北京          |
|        2 |        1001 | 2023-03-01 | 200.00 |        NULL | NULL          | NULL          | NULL          |
|        3 |          11 | NULL       |  50.00 |          11 | Smith         | Smith         | 上海          |
|        4 |        NULL | 2023-04-01 |   NULL |        NULL | NULL          | NULL          | NULL          |
|     NULL |        NULL | NULL       |   NULL |        NULL | NULL          | NULL          | NULL          |
+----------+-------------+------------+--------+-------------+---------------+---------------+---------------+
6 rows in set (0.00 sec)

附录：常用的 SQL 标准有哪些

在正式开始讲连接表的种类时，我们首先需要知道 SQL 存在不同版本的标准规范，因为不同规范下的表连接操作是有区别的。

SQL 有两个主要的标准，分别是 SQL92 和 SQL99。92 和 99 代表了标准提出的时间，SQL92 就是 92 年提出的标准规范。当然除了 SQL92 和 SQL99 以外，还存在 SQL-86、SQL-89、SQL:2003、SQL:2008、SQL:2011 和 SQL:2016 等其他的标准。

这么多标准，到底该学习哪个呢？实际上最重要的 SQL 标准就是 SQL92 和 SQL99。一般来说 SQL92 的形式更简单，但是写的 SQL 语句会比较长，可读性较差。而 SQL99 相比于 SQL92 来说，语法更加复杂，但可读性更强。我们从这两个标准发布的页数也能看出，SQL92 的标准有 500 页，而 SQL99 标准超过了 1000 页。实际上从 SQL99 之后，很少有人能掌握所有内容，因为确实太多了。就好比我们使用 Windows、Linux 和 Office 的时候，很少有人能掌握全部内容一样。我们只需要掌握一些核心的功能，满足日常工作的需求即可。

SQL92 和 SQL99 是经典的 SQL 标准，也分别叫做 SQL-2 和 SQL-3 标准。也正是在这两个标准发布之后，SQL 影响力越来越大，甚至超越了数据库领域。现如今 SQL 已经不仅仅是数据库领域的主流语言，还是信息领域中信息处理的主流语言。在图形检索、图像检索以及语音检索中都能看到 SQL 语言的使用。

参考：
https://www.bilibili.com/video/BV1iq4y1u7vj/?spm_id_from=333.337.search-card.all.click&vd_source=661c82170c96dfe1717bbda84ba2a36f

posted @ 2023-09-02 12:28 言非阅读(58) 评论(0) 编辑收藏举报

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言非の博客

机器学习、复杂网络

多表查询

一、一个案例引发的多表连接

1. 案例说明

2. 笛卡尔积（或交叉连接）的理解

3. 案例分析与问题解决

二、多表查询分类

1. 等值连接 vs 非等值连接

2. 自连接 vs 非自连接

3. 内连接 vs 外连接

三、9种join方式

INNER JOIN

LEFT JOIN

RIGHT JOIN

LEFT JOIN WHERE IS NULL

RIGHT JOIN WHERE IS NULL

四、UNION的使用

UNION操作符

UNION ALL操作符

五、扩展：SQL99语法新特性

1.自然连接

2.USING连接

多表连接查询

附录：常用的 SQL 标准有哪些

公告

Loading

言非の博客

机器学习、复杂网络

多表查询

一、 一个案例引发的多表连接

1. 案例说明

2. 笛卡尔积（或交叉连接）的理解

3. 案例分析与问题解决

二、多表查询分类

1. 等值连接 vs 非等值连接

2. 自连接 vs 非自连接

3. 内连接 vs 外连接

三、9种join方式

INNER JOIN

LEFT JOIN

RIGHT JOIN

LEFT JOIN WHERE IS NULL

RIGHT JOIN WHERE IS NULL

四、UNION的使用

UNION操作符

UNION ALL操作符

五、扩展：SQL99语法新特性

1.自然连接

2.USING连接

多表连接查询

附录：常用的 SQL 标准有哪些

公告

一、一个案例引发的多表连接