mercy mercy - 1 month ago 6
MySQL Question

using left join along with 'IS NULL'

I came across this query to get a row having max value. Can you explain how this works

SELECT s1.article, s1.dealer, s1.price, s2.article, s2.dealer, s2.price
FROM shop s1
LEFT JOIN shop s2 ON s1.price < s2.price
WHERE s2.article IS NULL;


Dump :

CREATE TABLE `shop` (
`article` int(4) unsigned zerofill NOT NULL DEFAULT '0000',
`dealer` char(20) NOT NULL DEFAULT '',
`price` double(16,2) NOT NULL DEFAULT '0.00',
PRIMARY KEY (`article`,`dealer`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `shop` VALUES
(0001,'A',3.45), (0001,'B',3.99),
(0002,'A',10.99),(0003,'B',1.45),
(0003,'C',1.69),(0003,'D',1.25),
(0004,'D',19.95);

Answer

The query performs a self join based on the predicate:

s1.price < s2.price

So, each record of the table is joined to any record having a greater price.

The predicate of the WHERE clause:

WHERE s2.article IS NULL

is true when no such record exists. Hence the query returns the record having the greatest price.