Question

我是SQL开发的新手。所以我想从SO获得一些帮助。

我有三张桌子：学生， student_addresses ， student_phones 。

他们的架构大致如下：

student
-------
student_id (Primary Key)
student_name
father_name
mother_name

student_addresses
-----------------
student_address_id (Primary Key)
student_id (Foreign Key)
address
zip_code

student_phones
--------------
student_phone_id (Primary Key)
student_id (Foreign Key)
phone_type
phone_number

student_addresses 和 student_phones 都是has_many关系。所以我想选择学生中的所有字段来获取特定的 student_id ，但只选择来自student_addresses和 student_phones 的匹配计数（总计）的 student_id数据即可。我怎么做到的？

我已尝试过此查询，但会返回错误：

SELECT students.student_id,student_name,father_name,mother_name,
       COUNT(student_addresses.student_id) AS total_addresses,    
       COUNT(student_phones.student_id) AS total_phones
 FROM students,student_phones,student_addresses
 WHERE students.student_id = student_phones.student_id AND
       students.student_id = student_addresses.student_id AND
       students.student_id = 7;

PS：目前我在PostgreSQL上使用它。但是，我也想在MySQL上工作。那么这是否意味着我需要有两个不同的查询？ AFAIK，为此目的，只有一个查询可以同时工作（因为MySQL和PostgreSQL都遵循相同的SQL实现，就此查询要求而言）。

我想知道，如果我可以不使用GROUP BY那么做。因为，假设学生表有更多的字段，比如12，那么我将把所有的字段名都放在SELECT和GROUP BY（AFAIK）上，这看起来有点不雅。

Answer 1

只需添加GROUP BY：

SELECT students.student_id,student_name,father_name,mother_name,
       COUNT(student_addresses.student_id) AS total_addresses,    
       COUNT(student_phones.student_id) AS total_phones
 FROM students,student_phones,student_addresses
 WHERE students.student_id = student_phones.student_id AND
       students.student_id = student_addresses.student_id AND
       students.student_id = 7
 GROUP BY students.student_id,student_name,father_name,mother_name;

但如果发生id为7的学生没有地址或没有电话号码，则不会返回任何结果。要在这种情况下返回某些内容，请尝试使用LEFT JOIN s：

SELECT students.student_id,student_name,father_name,mother_name,
       COUNT(student_addresses.student_id) AS total_addresses,    
       COUNT(student_phones.student_id) AS total_phones
 FROM students
 LEFT JOIN student_phones ON students.student_id = student_phones.student_id
 LEFT JOIN student_addresses ON students.student_id = student_addresses.student_id
 WHERE students.student_id = 7
 GROUP BY students.student_id,student_name,father_name,mother_name;

Answer 2

您忘记加入GROUP BY

SELECT students.student_id,student_name,father_name,mother_name,
           COUNT(student_addresses.student_id) AS total_addresses,    
           COUNT(student_phones.student_id) AS total_phones
     FROM students,student_phones,student_addresses
     WHERE students.student_id = student_phones.student_id AND
           students.student_id = student_addresses.student_id AND
           students.student_id = 7
    GROUP BY BY students.student_id,student_name,father_name,mother_name;

Answer 3

这适用于MySQL和PostgreSQL：

SELECT s.student_id,
       max(s.student_name) student_name,
       max(s.father_name) father_name,
       max(s.mother_name) mother_name,
       COUNT(distinct a.student_address_id) total_addresses,    
       COUNT(distinct p.student_phone_id) total_phones
FROM students s
LEFT JOIN student_phones p ON s.student_id = p.student_id
LEFT JOIN student_addresses a ON s.student_id = a.student_id
WHERE s.student_id = 7
GROUP BY s.student_id

SQL Query获取数据

3 个答案: