如何在SQL中有效地计算列值的出现次数？

Question

我有一张学生桌:

id | age
--------
0  | 25
1  | 25
2  | 23

我想查询所有学生,还有一个额外的专栏,用于计算同一年龄段的学生人数:

id | age | count
----------------
0  | 25  | 2
1  | 25  | 2
2  | 23  | 1

这样做最有效的方法是什么？我担心子查询会很慢,我想知道是否有更好的方法.在那儿？

Answer 1

这应该工作:

SELECT age, count(age) 
  FROM Students 
 GROUP by age

如果您还需要id,则可以将上面的内容包含在子查询中,如下所示:

SELECT S.id, S.age, C.cnt
  FROM Students  S
       INNER JOIN (SELECT age, count(age) as cnt
                     FROM Students 
                    GROUP BY age) C ON S.age = C.age

Answer 2

如果您使用的是Oracle,那么称为分析的功能就可以实现.它看起来像这样:

select id, age, count(*) over (partition by age) from students;

如果您不使用Oracle,那么您需要重新加入计数:

select a.id, a.age, b.age_count
  from students a
  join (select age, count(*) as age_count
          from students
         group by age) b
    on a.age = b.age

Answer 3

这是另一种解决方案.这个使用非常简单的语法.接受的解决方案的第一个示例不适用于旧版本的Microsoft SQL(即2000)

SELECT age, count(*)
FROM Students 
GROUP by age
ORDER BY age

Answer 4

我会做的事情如下:

select
 A.id, A.age, B.count 
from 
 students A, 
 (select age, count(*) as count from students group by age) B
where A.age=B.age;

Answer 5

select s.id, s.age, c.count
from students s
inner join (
    select age, count(*) as count
    from students
    group by age
) c on s.age = c.age
order by id