mysql的inner join 和left join区别

1. INNER JOIN

INNER JOIN 只返回两个表中满足连接条件的匹配行。换句话说，它只返回那些在连接的两个表中都有匹配值的行。如果某一行在其中一个表中没有匹配项，那么这行不会出现在结果集中。

写法：

sql 复制代码

SELECT columns
FROM table1
INNER JOIN table2
ON table1.column = table2.column;

假设有两个表 employees 和 departments：

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employees:
+----+----------+------------+
| id | name     | dept_id    |
+----+----------+------------+
| 1  | Alice    | 1          |
| 2  | Bob      | 2          |
| 3  | Charlie  | NULL       |
+----+----------+------------+

departments:
+----+--------------+
| id | department   |
+----+--------------+
| 1  | HR           |
| 2  | Engineering  |
| 3  | Marketing    |
+----+--------------+

执行 INNER JOIN：

sql 复制代码

SELECT employees.name, departments.department
FROM employees
INNER JOIN departments
ON employees.dept_id = departments.id;

结果为：

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+--------+--------------+
| name   | department   |
+--------+--------------+
| Alice  | HR           |
| Bob    | Engineering  |
+--------+--------------+

2. LEFT JOIN

LEFT JOIN 返回左表中的所有行，即使在右表中没有匹配项。在这种情况下，右表中的结果为 NULL。

用法：

sql 复制代码

SELECT columns
FROM table1
LEFT JOIN table2
ON table1.column = table2.column;

同上文中提到的表，执行以下SQL

sql 复制代码

SELECT employees.name, departments.department
FROM employees
LEFT JOIN departments
ON employees.dept_id = departments.id;

执行结果如下：

bash 复制代码

+--------+--------------+
| name   | department   |
+--------+--------------+
| Alice  | HR           |
| Bob    | Engineering  |
| Charlie| NULL         |
+--------+--------------+