使用COUNT執行2個表的INNER JOIN（）

Question

我被困在SQL查詢中，我希望你們能幫助我。

我有2個表：

EVENTS 
event_id (PK) 
event_name 

ORDERS 
order_id (PK) 
event_id (FK) 

，我試圖執行這些表以下查詢：

SELECT 
    e.event_id, 
    e.name, 
    COUNT(o.event_id) AS booked  
FROM 
    events AS e 
INNER JOIN 
    orders AS o 
ON 
    e.event_id = o.event_id 
WHERE 
    e.event_id IN (1, 2, 3) 

的問題是，我得到的結果是：

+----------+------+--------+ 
| event_id | name | booked | 
+----------+------+--------+ 
|  NULL | NULL |  0 | 
+----------+------+--------+ 

但是當我運行3項獨立的查詢與：

WHERE e.event_id IN (1) 

WHERE e.event_id IN (2) 

WHERE e.event_id IN (3) 

我得到的結果我想：

+----------+-------+--------+ 
| event_id | name | booked | 
+----------+-------+--------+ 
|  1 | Test1 |  0 | 
+----------+-------+--------+ 

+----------+-------+--------+ 
| event_id | name | booked | 
+----------+-------+--------+ 
|  2 | Test2 |  0 | 
+----------+-------+--------+ 

+----------+-------+--------+ 
| event_id | name | booked | 
+----------+-------+--------+ 
|  3 | Test3 |  0 | 
+----------+-------+--------+ 

我在做什麼錯？有沒有辦法只使用一個查詢，並得到：

+----------+-------+--------+ 
| event_id | name | booked | 
+----------+-------+--------+ 
|  1 | Test1 |  0 | 
+----------+-------+--------+ 
|  2 | Test2 |  0 | 
+----------+-------+--------+ 
|  3 | Test3 |  0 | 
+----------+-------+--------+ 

請幫忙。

UPDATE： 當我運行：

SELECT 
    e.event_id, 
    e.name, 
    COUNT(o.event_id) AS booked  
FROM 
    events AS e 
LEFT JOIN 
    orders AS o 
ON 
    e.event_id = o.event_id 
WHERE 
    e.event_id IN (1, 2, 3) 

我只得到：

+----------+-------+--------+ 
| event_id | name | booked | 
+----------+-------+--------+ 
|  1 | Test1 |  0 | 
+----------+-------+--------+ 

Answer 1

您應該使用LEFT JOIN代替INNER JOIN，不要忘記，因爲你正在使用使用GROUP BY條款集合函數COUNT()。

SELECT e.event_id, 
     e.name, 
     COUNT(o.event_id) AS booked  
FROM events AS e 
     LEFT JOIN orders AS o 
      ON e.event_id = o.event_id 
WHERE e.event_id IN (1, 2, 3) 
GROUP BY e.event_id, e.name 

• SQLFiddle Demo

爲了進一步獲得更多的知識有關加入，請訪問以下鏈接：

• Visual Representation of SQL Joins

Answer 2

使用留下了一個if條件加盟可以做到

SELECT 
    e.event_id, 
    e.name, 
    SUM(IF(o.event_id IS NULL, 0, 1)) AS booked 
FROM events AS e 
    LEFT JOIN orders AS o 
    ON e.event_id = o.event_id 
WHERE e.event_id IN(1, 2, 3) 
GROUP BY e.event_id, e.name 

Fiddle Demo

或者你可以像這樣做太

SELECT 
    e.event_id, 
    e.name, 
    COUNT(o.event_id) AS booked 
FROM events AS e 
    LEFT JOIN orders AS o 
    ON e.event_id = o.event_id 
WHERE e.event_id IN(1, 2, 3) 
GROUP BY e.event_id, e.name 

SQL Fiddle Demo