Question

SQL FIDDLE：http://sqlfiddle.com/#!2/5895f/5

我有群组，每个群组（ item_group ）可以包含多个项目（项目），每个项目每个用户都有一个状态条目（ item_status ）。我正在尝试检索不属于您自己的每个组的最后一项。可以把它想象成一个消息线程，你只想看到该线程的预览是发送的最后一条消息，但不是你的消息。

这适用于我的查询

  SELECT * FROM (
    SELECT it.id as group_id, i.sender_id as sender_id
    FROM item_group it
    JOIN item i ON (i.item_group_id = it.id)
    JOIN item_status s ON (s.item_id = i.id)
  ) as results
  WHERE results.sender_id != 2  
  GROUP BY results.group_id;

然而，下一步是我想确保我的项目的任何项目的状态不是0这是我的问题所在。它表示未知列'results.group_id'

  SELECT * FROM (
    SELECT it.id as group_id, i.sender_id as sender_id
    FROM item_group it
    JOIN item i ON (i.item_group_id = it.id)
    JOIN item_status s ON (s.item_id = i.id)
  ) as results
  WHERE results.sender_id != 2
  AND (SELECT COUNT(*) = 0 
       FROM item_status s
       --- PROBLEM IS HERE results.group_id ---
       JOIN item i ON (i.item_group_id = results.group_id AND s.item_id = i.id) 
       ---------------------------------------- 
       WHERE s.user_id = 2 AND s.status = 0)
  GROUP BY results.group_id;



 CREATE TABLE item_group (
  id      int(10) PRIMARY KEY NOT NULL AUTO_INCREMENT);

  CREATE TABLE item (
  id             int(10) PRIMARY KEY NOT NULL AUTO_INCREMENT, 
  item_group_id  int(10) NOT NULL, 
  sender_id MEDIUMINT UNSIGNED NOT NULL);

  CREATE TABLE item_status (
  item_id     int(10) NOT NULL, 
  user_id     MEDIUMINT UNSIGNED NOT NULL, 
  status      int(10) NOT NULL);

  INSERT INTO item_group (ID) VALUES (1);
  INSERT INTO item (item_group_id, sender_id) VALUES (1, 1);
  INSERT INTO item (item_group_id, sender_id) VALUES (1, 2);
  INSERT INTO item_status (item_id, user_id, status) VALUES (1, 1, 1);
  INSERT INTO item_status (item_id, user_id, status) VALUES (1, 2, 1);
  INSERT INTO item_status (item_id, user_id, status) VALUES (2, 1, 1);
  INSERT INTO item_status (item_id, user_id, status) VALUES (2, 2, 1);

Answer 1

我会用以下方式写这个，没有子查询：

SELECT i.item_group_id as group_id, i.sender_id as sender_id
FROM item i 
JOIN item_status s ON (s.item_id = i.id)
LEFT OUTER JOIN (
    item i2 INNER JOIN item_status s2 
      ON (s2.item_id = i2.id AND s2.user_id = 2 AND s2.status = 0) 
) ON (i.item_group_id = i2.item_group_id)
WHERE i.sender_id != 2 AND i2.item_group_id IS NULL
GROUP BY group_id ORDER BY NULL;

您还应该创建以下索引：

ALTER TABLE item_status 
    ADD KEY (user_id, status),
    ADD KEY (item_id);

我在MySQL 5.5.30上测试了这个查询。使用推荐的索引，优化程序EXPLAIN报告如下：

+----+-------------+-------+--------+-----------------+---------+---------+-----------------+------+----------------------------------------------+
| id | select_type | table | type   | possible_keys   | key     | key_len | ref             | rows | Extra                                        |
+----+-------------+-------+--------+-----------------+---------+---------+-----------------+------+----------------------------------------------+
|  1 | SIMPLE      | i     | ALL    | PRIMARY         | NULL    | NULL    | NULL            |    2 | Using where; Using temporary                 |
|  1 | SIMPLE      | s2    | ref    | user_id,item_id | user_id | 7       | const,const     |    0 |                                              |
|  1 | SIMPLE      | i2    | eq_ref | PRIMARY         | PRIMARY | 4       | test.s2.item_id |    1 | Using where                                  |
|  1 | SIMPLE      | s     | ref    | item_id         | item_id | 4       | test.i.id       |    1 | Using index                                  |
+----+-------------+-------+--------+-----------------+---------+---------+-----------------+------+----------------------------------------------+

我想到了一个针对您的后续问题的快速解决方案，如何获取所有项目的状态为0的项目组。

SELECT i.item_group_id as group_id, i.sender_id as sender_id
FROM item i 
JOIN item_status s ON (s.item_id = i.id)
LEFT OUTER JOIN (
    item i2 INNER JOIN item_status s2 
      ON (s2.item_id = i2.id AND s2.user_id = 2 AND s2.status <> 0) 
) ON (i.item_group_id = i2.item_group_id)
WHERE i.sender_id != 2 AND i2.item_group_id IS NULL
GROUP BY group_id ORDER BY NULL;

除了我在连接条件中将= 0更改为<> 0之外，这与上述查询几乎完全相同。所以这意味着：

“尝试找到任何非零状态，如果找不到任何状态，那么OUTER JOIN将为连接表的所有列返回null。当发生这种情况时，你发现了一个包含所有列的项目组零状态。“

PS：我没有测试过这个，只是提供它作为快速解决方案。

在另一个嵌套查询中使用嵌套查询结果

1 个答案: