mysql 拾遗

联合查询 join

the main difference between JOIN and LEFT JOIN lies in how they retrieve data from tables:

• JOIN (INNER JOIN): Returns rows when there is a match in both tables. Excludes rows where there is no match between the tables.
• LEFT JOIN (LEFT OUTER JOIN): Returns all rows from the left table, even if there are no matches in the right table.

示例

假设有两个表：Employees 和 Departments。

Employees 表：

sql
+-----------+----------+------------+
| EmployeeID | Name     | DepartmentID |
+-----------+----------+------------+
| 1         | Alice    | 1          |
| 2         | Bob      | 2          |
| 3         | Charlie  | 3          |
+-----------+----------+------------+

Departments 表：

sql
+------------+------------+
| DepartmentID | DepartmentName |
+------------+------------+
| 1          | IT         |
| 2          | HR         |
| 4          | Marketing  |