获取一个SQL查询以打印0表示3个表中的空计数结果

Question

I'm trying to get a SQL query to give me the results of a count but I need the result to include rows where the count is 0. What I found for solutions to this was to use IFNULL(COUNT(*), 0) in place of COUNT(*) however that had no effect on the result. I also tried using a LEFT JOIN but SQL gave me a syntax error if I tried to put in those. Here's my table setup

User

+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| UserID      | mediumint(9) | NO   | PRI | NULL    | auto_increment |
| firstName   | varchar(15)  | NO   |     | NULL    |                |
| lastName    | varchar(15)  | NO   |     | NULL    |                |
| Protocol    | varchar(10)  | NO   |     | NULL    |                |
| Endpoint    | varchar(50)  | NO   |     | NULL    |                |
| UsergroupID | mediumint(9) | NO   | MUL | NULL    |                |
+-------------+--------------+------+-----+---------+----------------+

Subscription

+----------------+--------------+------+-----+---------+----------------+
| Field          | Type         | Null | Key | Default | Extra          |
+----------------+--------------+------+-----+---------+----------------+
| SubscriptionID | mediumint(9) | NO   | PRI | NULL    | auto_increment |
| TopicID        | mediumint(9) | NO   | MUL | NULL    |                |
| UserID         | mediumint(9) | NO   | MUL | NULL    |                |
+----------------+--------------+------+-----+---------+----------------+

Topic

+----------+--------------+------+-----+---------+----------------+
| Field    | Type         | Null | Key | Default | Extra          |
+----------+--------------+------+-----+---------+----------------+
| TopicID  | mediumint(9) | NO   | PRI | NULL    | auto_increment |
| Name     | varchar(50)  | NO   |     | NULL    |                |
| FBName   | varchar(30)  | YES  |     | NULL    |                |
| FBToken  | varchar(255) | YES  |     | NULL    |                |
| TWName   | varchar(10)  | YES  |     | NULL    |                |
| TWToken  | varchar(50)  | YES  |     | NULL    |                |
| TWSecret | varchar(50)  | YES  |     | NULL    |                |
+----------+--------------+------+-----+---------+----------------+

My SQL query to try and get the COUNT is :

SELECT Topic.TopicID as ID, Topic.Name AS TopicName, COUNT(*) AS numSubscriptions
FROM User, Topic, Subscription
WHERE Subscription.UserID = User.UserID
  AND Subscription.TopicID = Topic.TopicID
GROUP BY Topic.TopicID;

I've tried replacing COUNT(*) with IFNULL(COUNT(*), 0) and I've tried to replace User,Topic,Subscription with User JOIN Subscription JOIN Topic and I also tried User LEFT JOIN Subscription LEFT JOIN Topic but that got a SQL error.

The output I'm getting is:

+----+-----------+------------------+
| ID | TopicName | numSubscriptions |
+----+-----------+------------------+
|  2 | test      |                2 |
|  3 | test2     |                1 |
+----+-----------+------------------+

I need to be getting

+----+-----------+------------------+
| ID | TopicName | numSubscriptions |
+----+-----------+------------------+
|  2 | test      |                2 |
|  3 | test2     |                1 |
|  4 | test3     |                0 |
+----+-----------+------------------+

Answer 1

By default, outer joins are left to right. So, the trick is to start with Topic:

SELECT Topic.TopicID as ID, Topic.Name AS TopicName,
  COUNT(User.UserID) AS numSubscriptions
FROM Topic
LEFT JOIN Subscription
  ON Subscription.TopicID = Topic.TopicID
JOIN User
  ON User.UserID = Subscription.UserID
GROUP BY Topic.TopicID

This allows for multiple subscriptions per user and requires that the user record exists to be considered in the count.

COUNT(NULL) evaluates to 0 , so any topic records without a corresponding subscription and user record will show as 0 .

If you're not concerned whether the user record exists, you could simplify it to the following:

SELECT Topic.TopicID as ID, Topic.Name AS TopicName,
  COUNT(Subscription.TopicID) AS numSubscriptions
FROM Topic
LEFT JOIN Subscription
  ON Subscription.TopicID = Topic.TopicID
GROUP BY Topic.TopicID

Answer 2

The example below should do what you're after. The column in the COUNT() can be any column of the subscription table, but using its ID is a good practice.

Using the left join ensures that all entries of the user table will show up in the results, even if there are no matching subscriptions.

SELECT User.firstName,
       User.lastName,
       Topic.Name AS TopicName,
       COUNT(Subscription.SubscriptionId) AS numSubscriptions
FROM USER
LEFT OUTER JOIN Subscription ON Subscription.UserID=USER.UserID
LEFT OUTER JOIN Topic ON Subscription.TopicID=Topic.TopicID
GROUP BY User.firstName, User.lastName, Topic.Name;