SQL查詢獲取數據

Question

我是SQL開發新手。所以我想從SO獲得一些幫助。

我有三個表：學生，student_addresses，student_phones。

其架構大致如下：

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 

兩個student_addresses和student_phones是的has_many關係。所以我想從學生選擇所有字段特定student_id數據但僅從student_addresses匹配的次數（總）和student_phones爲student_id數據。我如何獲得？

我已經試過此查詢，但它返回一個錯誤：

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：目前我使用這個在PostgreSQL上。不過，我也想在MySQL上工作。那麼這是否意味着我需要有兩個不同的查詢？ AFAIK，爲此目的，只有一個查詢可以同時工作（因爲MySQL和PostgreSQL遵循相同的SQL實現，就這個查詢要求而言）。

我想知道，如果我可以做到這一點，而不使用GROUP BY。因爲，假設學生表有更多的字段，比如說12，那麼我將不得不將所有的字段名稱都放到SELECT中，以及放在GROUP BY（AFAIK）中，這似乎有點不雅觀。

Answer 1

這應該在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; 

但如果它發生學生ID爲7沒有地址或沒有電話號碼，它不會返回任何結果。即使在這種情況下返回的東西，嘗試使用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;