mysql实现row_number() over partition by
时间: 2023-08-30 13:10:28 浏览: 47
MySQL中没有内置的ROW_NUMBER() OVER(PARTITION BY)函数,但是可以使用变量和子查询的组合来实现类似的功能。下面是一个例子:
假设有一个表格students,包含以下字段:id, name, score, subject。要求对每个学科(subject)内的成绩按照分数降序排列,并给每个学科内的学生编号(从1开始)。
可以使用如下SQL语句:
```
SELECT id, name, score, subject,
@row_number:=CASE
WHEN @subject=subject THEN @row_number+1
ELSE 1
END AS row_number,
@subject:=subject AS dummy
FROM students
ORDER BY subject, score DESC;
```
这个语句中,使用了变量@row_number和@subject,它们的初始值都为NULL。在SELECT语句中,先根据学科和分数排序,然后对每个学科内的成绩赋予编号。如果当前处理的学科和上一行不同,则将@row_number重置为1,否则将其加1。同时使用一个dummy列来触发@subject的更新。最后将结果按照学科和行号排序即可。
相关问题
mysql中row_number() over(partition by
mysql中的row_number() over(partition by column order by column)是一种用于分组排序的功能。它可以按照指定的列进行分组,并按照指定的列进行排序,在每个分组内为每行分配一个行号。
下面是一个使用row_number() over(partition by column order by column)的示例代码:
select * from (
select *, row_number() over(partition by class order by score desc) as row_num
from table_name
) as result
where row_num = 1
以上代码将根据class列对数据进行分组,并按照score列进行降序排序。然后,使用row_number()函数为每个分组内的行分配行号。最后,通过where子句选择行号为1的行,即每个分组内得分最高的行。
mysql row_number() over(partition by )
MySQL does not have a built-in ROW_NUMBER() function like some other database systems (e.g., Oracle, SQL Server, PostgreSQL). However, you can achieve similar results using user-defined variables. Here's an example of how you can use user variables to generate row numbers within partitions:
```sql
SELECT
*,
@row_number := IF(@partition = partition_column, @row_number + 1, 1) AS row_number,
@partition := partition_column
FROM
your_table
ORDER BY
partition_column, ordering_column;
```
In the above query, `partition_column` is the column used for partitioning, and `ordering_column` is the column used for determining the order within each partition. Please replace `your_table` with the actual name of your table.
The query uses two user-defined variables: `@row_number` and `@partition`. The `IF` statement checks if the current row belongs to the same partition as the previous row (`@partition = partition_column`). If it does, it increments the row number (`@row_number + 1`). Otherwise, it resets the row number to 1. Finally, it updates the `@partition` variable with the partition value of the current row for comparison in the next iteration.
Keep in mind that user-defined variables have some caveats and limitations, so make sure to test and verify the results for your specific use case.