Sammenføj to tabeller, hvor tabel A har en datoværdi og skal finde den næste dato i B under datoen i A

-- Create and fill first table
CREATE TABLE `id_date` (
  `id` int(11) NOT NULL auto_increment,
  `iddate` date NOT NULL,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

INSERT INTO `id_date` VALUES(1, '2010-01-13');
INSERT INTO `id_date` VALUES(2, '2011-04-19');
INSERT INTO `id_date` VALUES(3, '2011-05-07');

-- Create and fill second table    
CREATE TABLE `date_val` (
  `mydate` date NOT NULL,
  `myval` varchar(4) collate utf8_bin NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

INSERT INTO `date_val` VALUES('2009-03-29', '0.5');
INSERT INTO `date_val` VALUES('2010-01-30', '0.55');
INSERT INTO `date_val` VALUES('2011-08-12', '0.67');

-- Get the result table as asked in question
SELECT iddate, t2.mydate, t2.myval
FROM `id_date` t1
JOIN date_val t2 ON t2.mydate <= t1.iddate
AND t2.mydate = ( 
SELECT MAX( t3.mydate ) 
FROM `date_val` t3
WHERE t3.mydate <= t1.iddate )

Hvad vi laver:

• for hver dato i id_date tabel (din tabel A ),
• vi finder datoen i date_val tabel (din tabel B )
• som er den højeste dato i date_val tabel (men stadig mindre end id_date.date)