Join two tables where table A has a date value and needs to find the next date in B below the date in A

Question

I got this table \"A\":

| id | date       |
===================
| 1  | 2010-01-13 |
| 2  | 2011-04-19 |
| 3  | 2011-05-07 |
| .. | ...        |

Answer 1

-- 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 )

What we're doing:

• for each date in the id_date table (your table A),
• we find the date in the date_val table (your table B)
• which is the highest date in the date_val table (but still smaller than the id_date.date)

Answer 2

The INNER JOIN return rows when there is at least one match in both tables. Try this.

Select A.id,A.date,b.value 
from A inner join B 
on A.date=b.date 

Answer 3

You could use a subquery with limit 1 to look up the latest value in table B:

select  id
,       date
,       (
        select  value
        from    B
        where   B.date < A.date
        order by
                B.date desc
        limit   1
        ) as value
from    A