MySQL parallel ranking and sequential ranking query
Prepare.
创建一张叫scores的表,内容如下。因为测试排名,所以就用最简单的结构。
id | score |
---|---|
99 | |
80 | |
87 | |
60 | |
80 | |
##99 |
获取分数排名,要求并列排名。如果两个分数相同,则两个分数排名(rank)相同。名次之间不应该有“间隔”。
The results are as follows.
rank | ||
---|---|---|
1 | 6 | |
1 | 3 | |
2 | 2 | |
3 | 5 | |
3 | 4 | |
4 |
按照上面的需求,我们可以知道我们是要做一个按照分数(score)查询的一个功能,只不过是要给排序好的结果加上一个我们想要的名次。
我们笨想,我们要想知道某个分数排第几名,是不是知道有几个比它大就行了。如果有零个比它大的,那么它就是第一名,如果只有一个比它大,
那么它就是第二名。以此类推就好了。
那么我们来分析上面的sql语句。它就是把socres表分成了俩个一样的表,a 表,b表。然后通过子查询去查rank的值。
第一步:select id,score, rank from scores order by rank;我们查询我们要的信息,但是我们scores表中没有rank这个字段,所以就要分成俩个一
样的表,做子查询,来查rank。
第二步:select id,score,(select count(score) from scores as b where b.score > a.score) + 1 as rank from scores as a order by rank;上面说过了
如果0个比某分数大,那么它就是第一名。所以我们要再查询的个数上加1。结果如下:
Copy after login
按照上面的需求,我们可以知道我们是要做一个按照分数(score)查询的一个功能,只不过是要给排序好的结果加上一个我们想要的名次。 我们笨想,我们要想知道某个分数排第几名,是不是知道有几个比它大就行了。如果有零个比它大的,那么它就是第一名,如果只有一个比它大, 那么它就是第二名。以此类推就好了。 那么我们来分析上面的sql语句。它就是把socres表分成了俩个一样的表,a 表,b表。然后通过子查询去查rank的值。 第一步:select id,score, rank from scores order by rank;我们查询我们要的信息,但是我们scores表中没有rank这个字段,所以就要分成俩个一 样的表,做子查询,来查rank。 第二步:select id,score,(select count(score) from scores as b where b.score > a.score) + 1 as rank from scores as a order by rank;上面说过了 如果0个比某分数大,那么它就是第一名。所以我们要再查询的个数上加1。结果如下:
我们发现结果不是我们预期的。因为我们还没有去重。比87大的有俩个都是99,那么87的rank就是2+1=3,而我们要的排名连续不断的。所以用distinct 关键字去重。 第三步:select id, score, (select count(distinct(score)) from scores as b where b.score > a.score ) + 1 as rank from scores as a order by rank;
Sequential ranking expected results
顺序排名我们就按照score字段倒序查询即可,只不过是用msyql的变量去做rank。mysql中的变量是用‘@’跟上变量名称。@rowNum php中我们用$rowNum。mysql中赋值用 := 来赋值。(select @rowNum :=0) r 是给变量@rowNum一个初始值为0。这个很好理解。就是 按照我们要排名的字段倒序去查询,再用mysql变量给每一条结果加一个排列序号。
sql statement
select t.id, t.score,@rowNum := @rowNum +1 as rank from (select @rowNum :=0) r, scores as t order by t.score desc ;
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