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我想获取给定的最新 MainNumber、Serial、BarType 和 Notes MainNumber（如果存在）。请注意，BarType 存储在查找表中并使用 BarID 进行引用。

Unreason 想出了这个：

SELECT @MainNumber, COALESCE(n.Notes, 'None')
FROM numbers 
     LEFT JOIN notes n ON numbers.MainNumber = n.MainNumber
     LEFT JOIN notes n2 ON n.MainNumber = n2.MainNumber AND n.Date < n2.Date
WHERE n2.Date IS NULL AND numbers.MainNumber = @MainNumber

无论 Notes 是否存在都NULL可以，但现在我需要 Serial 和 BarType。MainNumber 在其生命周期内可能已分配给多个序列，但我只想要最新的序列。（我需要对其他表中的大约 15 个其他字段执行此操作，因此在可能的情况下将不胜感激）

表

数字表：

CREATE TABLE `numbers` (
  `ID` int(10) unsigned NOT NULL auto_increment,
  `MainNumber` varchar(11) NOT NULL,
  `Serial` varchar(20) NOT NULL,
  `Date` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
  PRIMARY KEY  (`ID`),
  UNIQUE KEY `Serial` (`Serial`)
) ENGINE=MyISAM AUTO_INCREMENT=460 DEFAULT CHARSET=latin1

备注表：

CREATE TABLE `notes` (
  `ID` int(10) unsigned NOT NULL auto_increment,
  `MainNumber` varchar(11) NOT NULL,
  `Notes` longtext NOT NULL,
  `Date` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
  PRIMARY KEY  (`ID`),
  KEY `MainNumber` (`MainNumber`)
) ENGINE=MyISAM AUTO_INCREMENT=11 DEFAULT CHARSET=latin1 ROW_FORMAT=DYNAMIC

ref_bars 表：

CREATE TABLE `ref_bars` (
  `BarID` varchar(6) NOT NULL,
  `BarType` varchar(30) NOT NULL,
  PRIMARY KEY  USING BTREE (`BarID`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1

条形表：

CREATE TABLE `bars` (
  `ID` int(10) unsigned NOT NULL auto_increment,
  `MainNumber` varchar(11) NOT NULL,
  `BarID` varchar(6) NOT NULL,
  `Date` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
  PRIMARY KEY  (`ID`),
  KEY `MainNumber` (`MainNumber`)
) ENGINE=MyISAM AUTO_INCREMENT=212 DEFAULT CHARSET=latin1 ROW_FORMAT=DYNAMIC

样本数据

SELECT * FROM notes：

'ID','MainNumber','Notes','Date'
'1','1','repaired','2009-03-23 12:00:00'
'2','1','replaced','2009-08-15 19:20:05'

注意：MainNumber = 1 有两行，但 MainNumber 为 2 没有行。ID 只是技术性的，从未使用过。

SELECT * FROM numbers：

'ID','MainNumber','Serial','Date'
'1','1','4642785154854','2008-08-15 12:30:00'
'2','1','4642315642316','2009-08-15 12:50:00'
'3','2','5412558456223','2010-08-15 11:30:00'

SELECT * FROM bars：

'ID','MainNumber','BarID','Date'
'1','1',1,'2008-08-15 12:30:00'
'2','1',2,'2009-08-15 12:50:00'
'3','2',2,'2010-08-15 11:30:00'

SELECT * FROM ref_bars：

'BarID','BarType'
'1','Banned'
'2','Payment required'

预期产出

主号码 = 1

MainNumber,Notes,Banned,Unpaid
'1','replaced','Yes','Yes'

主号码 = 2

MainNumber,Notes,Banned,Unpaid
'2','None','No','Yes'

编辑：修复并测试它，同时使事情更清晰（希望如此）。今天早些时候我被赶去做其他事情，很抱歉用一个写得不好、不完整的问题浪费了人们的时间。

更新以阐明更复杂的要求

4

5 回答 5

1

选择忽略您的初始查询，因为它们不会重现您的问题我正在查看您的预期输出我假设您正在尝试实现以下目标：

给定MainNumber在表中查找记录notes并返回具有最大日期的行，如果给定的表注释中没有记录， MainNumber则返回常量 'None'

（这可能不是所要求的，所以如果不是，请更正预期的输出）

这可以很容易地实现，例如

SELECT @MainNumber, n.Notes
FROM notes n
     LEFT JOIN notes n2 ON n.MainNumber = n2.MainNumber AND n.Date < n2.Date
WHERE n2.Date IS NULL AND n.MainNumber = @MainNumber

这将返回笔记中的最新行。现在从应用程序端，如果它不返回任何行，只需打印 @MainNumber, 'None' 就是这样......

如果您寻找纯 SQL（并假设您确实需要 numbers 表中的一些其他列），那么您可以这样做：

SELECT @MainNumber, COALESCE(n.Notes, 'None')
FROM numbers 
     LEFT JOIN notes n ON numbers.MainNumber = n.MainNumber
     LEFT JOIN notes n2 ON n.MainNumber = n2.MainNumber AND n.Date < n2.Date
WHERE n2.Date IS NULL AND numbers.MainNumber = @MainNumber

编辑：第一个查询经过测试

mysql> SET @MainNumber = 1;                                                     Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @MainNumber, n.Notes FROM notes n      LEFT JOIN notes n2 ON n.MainNumber = n2.MainNumber AND n.Date < n2.Date WHERE n2.Date IS NULL AND n.MainNumber = @MainNumber;
+-------------+----------+
| @MainNumber | Notes    |
+-------------+----------+
|           1 | replaced | 
+-------------+----------+
1 row in set (0.00 sec)

如果 numbers 表中有多个条目，则第二个查询最初返回多行，DISTINCT 修复了该问题

mysql> SET @MainNumber = 1;                                                     Query OK, 0 rows affected (0.00 sec)

mysql> SELECT DISTINCT @MainNumber, COALESCE(n.Notes, 'None')
    -> FROM numbers 
    ->      LEFT JOIN notes n ON numbers.MainNumber = n.MainNumber
    ->      LEFT JOIN notes n2 ON n.MainNumber = n2.MainNumber AND n.Date < n2.Date
    -> WHERE n2.Date IS NULL AND numbers.MainNumber = @MainNumber
    -> ;
+-------------+---------------------------+
| @MainNumber | COALESCE(n.Notes, 'None') |
+-------------+---------------------------+
|           1 | replaced                  | 
+-------------+---------------------------+
1 row in set (0.00 sec)

mysql> SET @MainNumber = 2;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT DISTINCT @MainNumber, COALESCE(n.Notes, 'None')
    -> FROM numbers 
    ->      LEFT JOIN notes n ON numbers.MainNumber = n.MainNumber
    ->      LEFT JOIN notes n2 ON n.MainNumber = n2.MainNumber AND n.Date < n2.Date
    -> WHERE n2.Date IS NULL AND numbers.MainNumber = @MainNumber
    -> ;
+-------------+---------------------------+
| @MainNumber | COALESCE(n.Notes, 'None') |
+-------------+---------------------------+
|           2 | None                      | 
+-------------+---------------------------+
1 row in set (0.00 sec)
于 2010-10-11T15:21:40.443 回答
1

正如 Unreason 所建议的，您可以使用 JOIN 来完成。

另一种方法是使用子查询：

select distinct s.MainNumber, 
COALESCE(
  (select n.notes from notes n where n.MainNumber=s.MainNumber order by n.Date desc limit 1), 
  'None') as LastNote
from numbers s
WHERE s.MainNumber=?

请注意，使用 JOIN 的解决方案可能会或可能不会表现更好，您必须尝试一下。

另请注意，“LIMIT”是 MySQL 特定的（不是 ANSI SQL），因此如果您打算迁移到另一个 DBMS，请小心。

于 2010-10-11T15:42:52.203 回答
0

也许是这样的？

LEFT JOIN (select notes n
JOIN (
    SELECT
        MAX(Date) AS Date,
        MainNumber
    FROM notes
    GROUP BY MainNumber
) AS nx
    ON n.MainNumber = nx.MainNumber AND n.Date = nx.Date) n
ON notes.MainNumber = main.MainNumber

我删除了 where 子句，因为您没有描述为什么拥有它。如果你需要它，你可以再次插入它。

编辑：查询已更新，但仍然很难准确理解您想要什么。当我编写示例数据时，我的意思是每个表有 2-3 行以及预期的输出，如果您可以简化示例，那就更好了。希望能帮助到你。

于 2010-10-11T10:09:32.460 回答
0 
SELECT MainNumber,
       COALESCE(Notes,"None") as Notes,
       if(BarID=1,"Yes","No") as Banned,
       if(BarID=2,"Yes","No") as Unpaid,
       COALESCE(notes.Date,CURRENT_TIMESTAMP) as Date
FROM numbers LEFT JOIN notes USING (MainNumber)
     LEFT JOIN bars USING (MainNumber)
GROUP BY MainNumber,Date
HAVING Date=COALESCE(MAX(notes.Date),CURRENT_TIMESTAMP)
于 2010-10-14T06:57:43.953 回答
0

我终于解决了。它实际上比我想象的要简单得多。它也更接近我正在编写的原始查询。

SELECT
    s.MainNumber,
    n.Notes
FROM numbers s
JOIN (
    SELECT
        MAX(Date) AS Date,
        MainNumber
    FROM numbers
    WHERE MainNumber = 2
) AS sx
    ON s.MainNumber = sx.MainNumber AND s.Date = sx.Date

LEFT JOIN notes n
    ON s.MainNumber = n.MainNumber
LEFT JOIN (
    SELECT
            MAX(Date) AS Date,
            COALESCE(MainNumber,0) AS MainNumber
    FROM notes
    WHERE MainNumber = 2
) AS nx
    ON n.MainNumber = nx.MainNumber AND n.Date = nx.Date
于 2010-10-14T11:41:13.303 回答