文章目录
- [一、 一个案例引发的多表连接](#一、 一个案例引发的多表连接)
-
- [1. 案例说明](#1. 案例说明)
- [2. 笛卡尔积（或交叉连接）的理解](#2. 笛卡尔积（或交叉连接）的理解)
- [3. 案例分析与问题解决](#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](#1. 等值连接 vs 非等值连接)
- [2. 自连接 vs 非自连接](#2. 自连接 vs 非自连接)
- [3. 内连接 vs 外连接](#3. 内连接 vs 外连接)
- 三、9种join方式
-
- [INNER JOIN](#INNER JOIN)
- [LEFT JOIN](#LEFT JOIN)
- [RIGHT JOIN](#RIGHT JOIN)
- [LEFT JOIN WHERE IS NULL](#LEFT JOIN WHERE IS NULL)
- [RIGHT JOIN WHERE IS NULL](#RIGHT JOIN WHERE IS NULL)
- 四、UNION的使用
-
- UNION操作符
- [UNION ALL操作符](#UNION ALL操作符)
- 五、扩展：SQL99语法新特性
- 多表连接查询
- [附录：常用的 SQL 标准有哪些](#附录：常用的 SQL 标准有哪些)
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：
sql
#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：
mysqlSELECT 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：
mysql#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
sql
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。
sql
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
sql
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"
sql
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：
sql
CREATE TABLE customers (
id INT PRIMARY KEY,
customer_id INT,
customer_name VARCHAR(50)
);插入数据到 customers 表：
sql
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：
sql
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：
sql
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);
sql
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
sql
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。
sql
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。
sql
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
sql
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
sql
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：
mysql
SELECT column,... FROM table1
UNION [ALL]
SELECT column,... FROM table2UNION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
mysql
#方式1
SELECT * FROM employees WHERE email LIKE '%a%' OR department_id>90;
mysql
#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
mysql
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：
mysql
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：
mysql
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：
mysql
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：
mysql
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
sql
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
sql
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
sql
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。
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