使用DISTINCT时发生了什么？

Question

这是我的表格及其中包含的数据:

Table: first

+----------+------+
| first_id | data |
+----------+------+
|        1 |    5 |
|        2 |    6 |
|        3 |    7 |
|        4 |    6 |
|        5 |    7 |
|        6 |    5 |
|        7 |    7 |
|        8 |    6 |
|        9 |    5 |
|       10 |    7 |
+----------+------+

Table: second
+-----------+----------+----------+
| second_id | first_id | third_id |
+-----------+----------+----------+
|         1 |        1 |        2 |
|         2 |        2 |        3 |
|         3 |        3 |        4 |
|         4 |        4 |        2 |
|         5 |        5 |        3 |
|         6 |        6 |        4 |
|         7 |        7 |        2 |
|         8 |        8 |        2 |
|         9 |        9 |        4 |
|        10 |       10 |        4 |
+-----------+----------+----------+

我的目的是获得third_id按data字段排序的s 列表.现在,我为此运行了以下查询.

SELECT
    third_id, data
FROM 
    first f JOIN second s ON ( s.first_id = f.first_id )
ORDER BY 
    data ASC;

我得到了预期的以下结果.

+----------+------+
| third_id | data |
+----------+------+
|        4 |    5 |
|        2 |    5 |
|        4 |    5 |
|        2 |    6 |
|        3 |    6 |
|        2 |    6 |
|        2 |    7 |
|        4 |    7 |
|        4 |    7 |
|        3 |    7 |
+----------+------+

以下查询也按预期工作.

SELECT 
    third_id
FROM 
    first f JOIN second s ON ( s.first_id = f.first_id )
ORDER BY 
    data ASC;

与输出

+----------+
| third_id |
+----------+
|        4 |
|        2 |
|        4 |
|        2 |
|        3 |
|        2 |
|        2 |
|        4 |
|        4 |
|        3 |
+----------+

然后我跑了以下.

SELECT DISTINCT
    third_id
FROM 
    first f JOIN second s ON ( s.first_id = f.first_id )
ORDER BY 
    data ASC;

但是,在这里我得到了意想不到的结果:

+----------+
| third_id |
+----------+
|        2 |
|        3 |
|        4 |
+----------+

在这里,3必须是经过2和4,因为我的订购data场.我究竟做错了什么？或者我必须采取不同的策略.

注意:此方案发生在我的项目中.此处提供的表不属于原始数据库.它由我创建来解释问题.原始表包含数千行.如果您想试验数据,我正在插入数据库转储:

--
-- Table structure for table `first`
--

CREATE TABLE IF NOT EXISTS `first` (
  `first_id` int(11) NOT NULL AUTO_INCREMENT,
  `data` int(11) NOT NULL,
  PRIMARY KEY (`first_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=11 ;

--
-- Dumping data for table `first`
--

INSERT INTO `first` (`first_id`, `data`) VALUES
(1, 5),
(2, 6),
(3, 7),
(4, 6),
(5, 7),
(6, 5),
(7, 7),
(8, 6),
(9, 5),
(10, 7);
--
-- Table structure for table `second`
--

CREATE TABLE IF NOT EXISTS `second` (
  `second_id` int(11) NOT NULL AUTO_INCREMENT,
  `first_id` int(11) NOT NULL,
  `third_id` int(11) NOT NULL,
  PRIMARY KEY (`second_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=11 ;

--
-- Dumping data for table `second`
--

INSERT INTO `second` (`second_id`, `first_id`, `third_id`) VALUES
(1, 1, 2),
(2, 2, 3),
(3, 3, 4),
(4, 4, 2),
(5, 5, 3),
(6, 6, 4),
(7, 7, 2),
(8, 8, 2),
(9, 9, 4),
(10, 10, 4);

Answer 1

你可能想做类似的事情

SELECT third_id
FROM first JOIN second USING (first_id)
GROUP BY third_id
ORDER BY aggregatesomething(data)

那是min(data)或max(data)或什么。