解释下 @c:=if(@p=score,@c,@r) as rank

这是一条MySQL的SQL语句，用于计算排名。具体解释如下：

- @c：表示一个变量，用于存储当前的排名。
- if(@p=score,@c,@r)：表示一个条件语句，如果当前的分数等于上一个分数，则当前排名不变，否则当前排名为上一个排名加1。
- as rank：表示将计算出来的排名结果命名为“rank”。

整个语句的意思是，如果当前分数等于上一个分数，则当前排名不变，否则将当前排名设置为上一个排名加1，并将结果命名为“rank”。

相关问题

SELECT a.s_id, @i:=@i+1 AS i, -- @k:=(case when @score=a.sum_score then @k else @i end) as rank, a.sum_score AS score FROM (SELECT s_id,SUM(s_score) AS sum_score FROM score GROUP BY s_id ORDER BY sum_score DESC)a, (SELECT @k:=0,@i:=0,@score:=0)s

This is a MySQL query that selects the student ID (s_id), assigns a sequential number to each row (i), and calculates the rank of each student based on their sum of scores (sum_score).

The query uses a subquery to first group the scores by student ID and calculate the sum of scores for each student. This subquery is then joined with a variable initialization subquery that sets the initial values of @k, @i, and @score to 0.

The variable @k is used to keep track of the current rank while iterating over the rows. The variable @i is used to assign a sequential number to each row. The variable @score is used to compare the sum_score of the current row with the sum_score of the previous row.

The CASE statement is used to check if the sum_score of the current row is equal to the sum_score of the previous row. If they are equal, then the rank remains the same. If they are not equal, then the rank is updated to the current sequential number.

Here is a breakdown of the query:

SELECT 
  a.s_id,                    -- Select the student ID
  @i:=@i+1 AS i,             -- Assign a sequential number to each row
  @k:=(case when @score=a.sum_score then @k else @i end) as rank, -- Calculate the rank
  a.sum_score AS score       -- Select the sum of scores for each student
FROM 
  (SELECT s_id,SUM(s_score) AS sum_score FROM score GROUP BY s_id ORDER BY sum_score DESC) a, -- Subquery to calculate sum of scores for each student
  (SELECT @k:=0,@i:=0,@score:=0) s -- Subquery to initialize variables

Note that the use of variables in this query is not recommended, as it can lead to unexpected results if the variables are not reset properly. It is better to use a subquery or a window function to calculate the rank.

This is a MySQL query that selects the student ID (s_id), assigns a sequential number to each row (i), and calculates the rank of each student based on their sum of scores (sum_score). The query uses a subquery to first group the scores by student ID and calculate the sum of scores for each student. This subquery is then joined with a variable initialization subquery that sets the initial values of @k, @i, and @score to 0. The variable @k is used to keep track of the current rank while iterating over the rows. The variable @i is used to assign a sequential number to each row. The variable @score is used to compare the sum_score of the current row with the sum_score of the previous row. The CASE statement is used to check if the sum_score of the current row is equal to the sum_score of the previous row. If they are equal, then the rank remains the same. If they are not equal, then the rank is updated to the current sequential number. Here is a breakdown of the query: 复制 SELECT a.s_id, -- Select the student ID @i:=@i+1 AS i, -- Assign a sequential number to each row @k:=(case when @score=a.sum_score then @k else @i end) as rank, -- Calculate the rank a.sum_score AS score -- Select the sum of scores for each student FROM (SELECT s_id,SUM(s_score) AS sum_score FROM score GROUP BY s_id ORDER BY sum_score DESC) a, -- Subquery to calculate sum of scores for each student (SELECT @k:=0,@i:=0,@score:=0) s -- Subquery to initialize variables Note that the use of variables in this query is not recommended, as it can lead to unexpected results if the variables are not reset properly. It is better to use a subquery or a window function to calculate the rank. 翻译

这是一个MySQL查询，它选择学生ID（s_id），为每一行分配一个连续的数字（i），并根据学生得分总和（sum_score）计算每个学生的排名。查询使用一个子查询首先按学生ID分组并计算每个学生的得分总和。然后，将此子查询与变量初始化子查询连接，该子查询将@k，@i和@score的初始值设置为0。变量@k用于在迭代行时跟踪当前排名。变量@i用于为每行分配一个连续的数字。变量@score用于将当前行的sum_score与上一行的sum_score进行比较。CASE语句用于检查当前行的sum_score是否等于上一行的sum_score。如果它们相等，则排名保持不变。如果它们不相等，则将排名更新为当前的连续数字。以下是查询的详细说明：

SELECT 
    a.s_id, -- 选择学生ID
    @i:=@i+1 AS i, -- 为每一行分配一个连续的数字
    @k:=(case when @score=a.sum_score then @k else @i end) as rank, -- 计算排名
    a.sum_score AS score -- 选择每个学生的得分总和
FROM 
    (SELECT s_id,SUM(s_score) AS sum_score FROM score GROUP BY s_id ORDER BY sum_score DESC) a, -- 子查询计算每个学生的得分总和
    (SELECT @k:=0,@i:=0,@score:=0) s -- 变量初始化子查询

请注意，不建议在查询中使用变量，因为如果变量没有正确重置，可能会导致意外的结果。最好使用子查询或窗口函数来计算排名。