How to join MySQL tables on date range? - mysql

Question

I have a source table (TableA) that contains multiple records for each day. I need to left join (on the date field) it to TableB that contains a few records per year.
The problem is that TableA should be joined to the earliest record from TableB where the date from TableA <= the date from TableB.
CREATE TABLE IF NOT EXISTS `tableA` (
`id` int(6) unsigned NOT NULL,
`date` date NOT NULL,
`content` varchar(200) NOT NULL,
PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `tableA` (`id`, `date`, `content`) VALUES
('1', '2017-10-03', 'The earth is round.'),
('2', '2018-01-01', 'The earth is flat'),
('3', '2018-01-01', 'One hundred angels can dance on the head of a pin'),
('4', '2018-01-02', 'The earth is flat and rests on a bull\'s horn'),
('5', '2018-01-03', 'The earth is like a ball.');
CREATE TABLE IF NOT EXISTS `tableB` (
`date` date NOT NULL,
`content` varchar(200) NOT NULL,
PRIMARY KEY (`date`)
) DEFAULT CHARSET=utf8;
INSERT INTO `tableB` (`date`, `content`) VALUES
('2017-01-01', 'ONE'),
('2017-12-01', 'TWO'),
('2018-01-02', 'THREE'),
('2018-01-05', 'FOUR');
Based on the this SQLFiddle, I'm looking for the following result.
tableA.id | tableB.content
--------------------------
1 | TWO
2 | THREE
3 | THREE
4 | THREE
5 | FOUR

Answer 1

Here is one solution:
SELECT a.id, b.content
FROM TableA a
JOIN TableB b ON b.date = (
SELECT MIN(b2.date)
FROM TableB b2
WHERE b2.date >= a.date
);
I'm not sure whether this is the most efficient way, but it works.