mysql - 使用 COUNT() 跨 2 个表执行 INNER JOIN

Question

我被一个 SQL 查询卡住了，希望你们能帮帮我。

我有 2 个表:

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:当我运行时:

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

要进一步了解有关联接的更多信息，请访问以下链接:

• Visual Representation of SQL Joins