使用 COUNT() 跨 2 个表执行 INNER JOIN

Question

我遇到了一个 SQL 查询问题，希望你们能帮助我。

我有 2 张桌子：

EVENTS
event_id (PK)
event_name

ORDERS
order_id (PK)
event_id (FK)

我正在尝试对这些表执行以下查询：

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)

问题是我得到的结果是：

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

但是当我运行 3 个单独的查询时：

WHERE e.event_id IN (1)

WHERE e.event_id IN (2)

WHERE e.event_id IN (3)

我得到了我想要的结果：

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

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

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

我做错了什么？有没有办法只使用一个查询并获取：

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

请帮忙。

更新： 当我跑步时：

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

您应该使用LEFT JOIN 而不是INNER JOIN，并且不要忘记使用GROUP BY 子句，因为您正在使用聚合函数 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

如需进一步了解联接，请访问以下链接：

• Visual Representation of SQL Joins

Answer 2

使用带有 if 条件的左连接可以做到这一点

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

或者你也可以这样做

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