Question

如果有人可以帮助我，我会遇到一些SQL查询的麻烦。所以我有学生，参与，课程，地点，student_to_courses 这些表格。我需要让所有学生参与到特定课程中，并且我正在使用此查询

select 
  s.name as student_name, c.name as course_name, p.name as place_name
from 
  student_to_courses s2c, 
  students_participation sp, 
  students s,
  courses c,
  places p

where 
s.id = s2c.participation_id AND
s.id = sp.profile_id AND 
c.id = s2c.course_id AND
p.id = c.place_id = p.id AND
s2c.course_id = 1

这是sqlfiddle

但我需要计算学生在特定地方的次数，例如 student_name，course_name，place_name，| 3 | 。现在，我正在使用另一个查询，我每次在循环中调用它。但我想知道是否有办法只用单一查询来做到这一点。提前感谢所有想帮助我的人。

Answer 1

您可以执行COUNT()，然后对其余列进行GROUP BY：

select s.name as student_name, 
       c.name as course_name, 
       p.name as place_name, 
       count(*)
from students s
inner join student_to_courses s2c on s.id = s2c.participation_id
inner join courses c on c.id = s2c.course_id
inner join places p on p.id = c.place_id
inner join students_participation sp on s.id = sp.profile_id
where s2c.course_id = 1
group by s.name,c.name,p.name

我还将连接更改为显式连接，因为更容易阅读容易出错的错误。

sqlfiddle demo

Answer 2

试试这个：

select 
  s.name as student_name, c.name as course_name, 
  p.name as place_name, count(*) qtd
from 
  student_to_courses s2c, 
  students_participation sp, 
  students s,
  courses c,
  places p
where 
   s.id = s2c.participation_id AND
   s.id = sp.profile_id AND 
   c.id = s2c.course_id AND
   p.id = c.place_id = p.id AND
   s2c.course_id = 1
group by s.name, c.name, p.name

count(*)是AGGREGATE FUNCTION请参阅此链接以了解。

mysql查询计数字段

2 个答案: