Question

I want to get the latest MainNumber, Serial, BarType and Notes for a given MainNumber, if they exist. Note that BarType is stored in a lookup table and referenced with BarID.

Unreason came up with this:

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

This is fine whether Notes is NULL or not, but now I need the Serial and the BarType. A MainNumber may have been assigned to multiple Serials during its lifetime, but I only want the latest Serial. (I’ll need to do this with about 15 other fields in other tables, so a performant answer would be appreciated where possible)

Tables

Numbers Table:

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

Notes table:

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 table:

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

bars table:

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

Sample Data

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'

Note: two rows for MainNumber = 1, but no row for a MainNumber of 2. The IDs are just technical and are never used.

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'

Expected Output

MainNumber = 1

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

MainNumber = 2

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

Edit: Fixed it and tested it, whilst making things clearer (hopefully). I was rushed off to do other things earlier today, sorry for wasting people’s time with a badly-written, incomplete question.

Updated to clarify the more complex requirements

Answer 1

I finally worked it out. It’s actually a lot more simple than I thought. It’s closer to the original query I was writing, too.

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