在select查询中计数

Question

我有以下查询：

select exam_id,semester_id from marks where student_id=150  and semester_id<=1914;

它的输出是：

exam_id     |    semester_id

   27               1913
   68               1913
   64               1914 

我想找出类似semester_id的计数。

当我尝试mysql_num_rows时，它会显示3.

但我想要这样的输出

 semester_id 1913---------->count 2
 semester_id 1914---------->count 1

Answer 1

您只需使用GROUP BY：

SELECT semester_id, COUNT(*) as count
 FROM marks  
 WHERE student_id=150 
  AND semester_id<=1914
 GROUP BY semester_id;

Answer 2

试试这个

select semester_id,count(*) as cnt 
from marks 
where student_id=150 and semester_id<=1914 
group by semester_id;

Answer 3

使用Count()聚合函数即可实现此目的。

 SELECT semester_id, COUNT(exam_id)  FROM marks  
 WHERE student_id=150  AND semester_id<=1914
 GROUP BY semester_id;

Answer 4

Select semester_id, count(exam_id)
From marks
Group By semester_id;

如果您有需要，请使用：

count(distinct exam_id)

Answer 5

使用count（）

SELECT semester_id, COUNT(*) as count
FROM marks  
WHERE student_id=150 
AND semester_id<=1914
GROUP BY semester_id;