SQL查询获取数据

Question

我是SQL开发新手。所以我想从SO获得一些帮助。

我有三个表：学生，student_addresses，student_phones。

其架构大致如下：

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 

两个student_addresses和student_phones是的has_many关系。所以我想从学生选择所有字段特定student_id数据但仅从student_addresses匹配的次数（总）和student_phones为student_id数据。我如何获得？

我已经试过此查询，但它返回一个错误：

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：目前我使用这个在PostgreSQL上。不过，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遵循相同的SQL实现，就这个查询要求而言）。

我想知道，如果我可以做到这一点，而不使用GROUP BY。因为，假设学生表有更多的字段，比如说12，那么我将不得不将所有的字段名称都放到SELECT中，以及放在GROUP BY（AFAIK）中，这似乎有点不雅观。

Answer 1

这应该在MySQL和PostgreSQL工作：

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 

Answer 2

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; 

但如果它发生学生ID为7没有地址或没有电话号码，它不会返回任何结果。即使在这种情况下返回的东西，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 3

你忘了包括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;