Question

我坚持使用SQL查询，我希望你们可以帮助我。

我有两张桌子：

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 |
+----------+------+--------+

但是当我用：

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得到：

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

请帮忙。

更新：我跑的时候：

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，因为您使用的是聚合函数 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

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

使用COUNT（）在2个表中执行INNER JOIN

2 个答案: