MySQL 5.7 可以通过使用变量实现分组 row_number。实现方法如下：
SET @num := 0, @group := '';
SELECT *,
@num := IF(@group = group_col, @num + 1, 1) AS row_number,
@group := group_col AS dummy
FROM table_name
ORDER BY group_col, order_col;
其中，group_col 是要分组的列名，order_col 是排序的列名。
例如，假设有以下表格：
| id | name | department |
|---|---|---|
| 1 | Amy | HR |
| 2 | Bob | IT |
| 3 | Cate | HR |
| 4 | Dave | Marketing |
如果想按照 department 分组，并为每个组内的行添加一个序号，可以使用以下查询语句：
SET @num := 0, @dept := '';
SELECT *,
@num := IF(@dept = department, @num + 1, 1) AS row_number,
@dept := department AS dummy
FROM employee
ORDER BY department, name;
执行结果如下：
| id | name | department | row_number |
|---|---|---|---|
| 1 | Amy | HR | 1 |
| 3 | Cate | HR | 2 |
| 2 | Bob | IT | 1 |
| 4 | Dave | Marketing | 1 |
可以看到，同部门的员工被正确地分组，并且在组内根据 name 排序，同时每个组内的员工都获得了一个 row_number 序号。
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