The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables. [@via w3schools]

A SQL JOIN clause combines records from two or more tables in a database. It creates a set that can be saved as a table or used as is. A JOIN is a means for combining fields from two tables by using values common to each. ANSI standard SQL specifies four types of JOINs: INNER, OUTER, LEFT, and RIGHT. In special cases, a table (base table, view, or joined table) can JOIN to itself in a self-join. [@via wikipedia]

SQL Joins are helpful when we have to fetch data with a single query from two or more database tables.

Case:

I need to store news/article information. The news can also have comments and tags. News can be submitted/posted by multiple users.

According to the above scenario, I have created 5 tables and inserted some data into them :-

news - to store news
comments - to store comments for any particular news
tags - to store tags associated with any particular news
users - to store user information
tags_news – to store relationship between news and tags

–
– Table structure for table `comments`
–

CREATE TABLE `comments` (
`id` bigint(20) NOT NULL auto_increment,
`news_id` bigint(20) NOT NULL,
`user_id` bigint(20) NOT NULL,
`detail` text NOT NULL,
`status` tinyint(1) NOT NULL default '1',
`comment_date` int(11) NOT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`),
KEY `news_id` (`news_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;


–
– Dumping data for table `comments`
–

INSERT INTO `comments` (`id`, `news_id`, `user_id`, `detail`, `status`, `comment_date`) VALUES
(1, 1, 2, 'nice post :)', 1, 1273431296),
(2, 2, 2, 'hahaha', 1, 1273431420),
(3, 3, 2, '3123123', 1, 1273431452),
(4, 1, 2, 'thank You', 1, 1273431475),
(5, 2, 1, 'congratulations!', 1, 1273431500);

– ——————————————————–

–
– Table structure for table `news`
–

CREATE TABLE `news` (
`id` bigint(20) NOT NULL auto_increment,
`user_id` bigint(20) NOT NULL,
`title` text NOT NULL,
`detail` text NOT NULL,
`visit` int(11) NOT NULL default '0',
`status` tinyint(1) NOT NULL default '1',
`created_date` int(11) NOT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

–
– Dumping data for table `news`
–

INSERT INTO `news` (`id`, `user_id`, `title`, `detail`, `visit`, `status`, `created_date`) VALUES
(1, 1, 'Extra Content', 'fasdfasdf', 2, 1, 1273431296),
(2, 1, 'My My Question', 'this is jpt question.. :D', 16, 1, 1273431389),
(3, 2, 'Am I ram?', 'I am ram..yahoo !!', 3, 1, 1273431420);

– ——————————————————–

–
– Table structure for table `tags`
–

CREATE TABLE `tags` (
`id` bigint(20) NOT NULL auto_increment,
`name` varchar(255) NOT NULL,
`slug` varchar(255) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

–
– Dumping data for table `tags`
–

INSERT INTO `tags` (`id`, `name`, `slug`) VALUES
(1, 'sagarmatha', 'sagarmatha'),
(2, 'nepal', 'nepal'),
(3, 'gautam buddha', 'gautam-buddha'),
(4, 'testing', 'testing'),
(5, 'tags', 'tags'),
(6, 'tasty apple', 'tasty-apple'),
(7, 'banana', 'banana');

– ——————————————————–

–
– Table structure for table `tags_news`
–

CREATE TABLE `tags_news` (
`id` bigint(20) NOT NULL auto_increment,
`news_id` bigint(20) NOT NULL,
`tags_id` bigint(20) NOT NULL,
PRIMARY KEY (`id`),
KEY `news_id` (`news_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

–
– Dumping data for table `tags_news`
–

INSERT INTO `tags_news` (`id`, `news_id`, `tags_id`) VALUES
(1, 1, 1),
(2, 1, 2),
(3, 1, 3),
(4, 2, 4),
(5, 2, 5),
(6, 2, 6),
(7, 3, 6),
(8, 3, 7);

– ——————————————————–

–
– Table structure for table `users`
–

CREATE TABLE `users` (
`id` bigint(20) NOT NULL auto_increment,
`user_id` bigint(20) NOT NULL,
`firstname` varchar(100) NOT NULL,
`lastname` varchar(100) NOT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

–
– Dumping data for table `users`
–

INSERT INTO `users` (`id`, `user_id`, `firstname`, `lastname`) VALUES
(1, 1, 'Mukesh', 'Chapagain'),
(2, 2, 'Christopher', 'Gayle'),
(3, 3, 'Brian', 'Lara');

– ——————————————————–

Now, by using SQL JOINS, I can fetch data from these tables for different conditions and in different ways. Here follows the different SQL Queries with the magical JOIN statements:-

1) SELECT NEWS WITH COMMENTS COUNT FOR EACH NEWS


SELECT n.*, ifnull(c.count,0) AS comment FROM news AS n
LEFT JOIN
(SELECT COUNT(comments.id) AS count, comments.news_id FROM comments GROUP BY comments.news_id) AS c
ON n.id = c.news_id
ORDER BY n.created_date DESC;


2) SELECT NEWS POSTED BY ANY PARTICULAR USER (HERE, user_id = 1)


SELECT n.* FROM news AS n
INNER JOIN
users AS u
ON n.user_id = u.id
WHERE u.id = 1;


3) SELECT NEWS POSTED BY ANY PARTICULAR USER (WITH USER’S FIRSTNAME AND LASTNAME) (HERE, user_id = 1)


SELECT n.*, u.firstname AS firstname, u.lastname AS lastname FROM news AS n
INNER JOIN
users AS u
ON n.user_id = u.id
WHERE u.id = 1;


4) SELECT NEWS POSTED BY ANY PARTICULAR USER (WITH COMMENTS COUNT FOR EACH NEWS) (HERE, user_id = 1)


SELECT n.*, ifnull(c.count,0) AS comment, u.firstname AS firstname, u.lastname AS lastname FROM news AS n
LEFT JOIN
(SELECT COUNT(comments.id) AS count, comments.news_id FROM comments GROUP BY comments.news_id) AS c
ON n.id = c.news_id
INNER JOIN
users AS u
ON n.user_id = u.id
WHERE u.id = 1;


5) SELECT COMMENTS FOR ANY PARTICULAR NEWS (HERE, news_id =1)


SELECT c.* FROM comments AS c
INNER JOIN
news AS n
ON c.news_id = n.id
WHERE c.news_id = 1;


6) SELECT COMMENTS FOR ANY PARTICULAR NEWS (ALONG WITH USER INFORMATION) (HERE, news_id = 1)


SELECT c.*, u.firstname AS firstname, u.lastname AS lastname FROM comments AS c
INNER JOIN
news AS n
ON c.news_id = n.id
INNER JOIN
users AS u
ON c.user_id = u.id
WHERE c.news_id = 1;


7) SELECT TAGS FOR ANY PARTICULAR NEWS (HERE, news_id = 1)


SELECT t.* FROM tags AS t
INNER JOIN
(SELECT tags_news.news_id AS news_id, tags_news.tags_id FROM tags_news WHERE tags_news.news_id = 1) AS tn
ON t.id = tn.tags_id;


8) SELECT NEWS BY TAG NAME (HERE, tag = ‘nepal’)


SELECT n.*, t.name AS tag_name, t.slug AS tag_slug FROM news AS n
INNER JOIN
tags_news as tn
ON n.id = tn.news_id
INNER JOIN tags AS t
ON tn.tags_id = t.id
WHERE t.slug = 'nepal';


9) SELECT NEWS BY TAG NAME (ALONG WITH COMMENTS COUNT) (HERE, tag = ‘nepal’)


SELECT n.*, ifnull(c.count,0) AS comment, t.name AS tag_name, t.slug AS tag_slug, t.id AS tag_id FROM news AS n
INNER JOIN
tags_news AS tn
ON (n.id = tn.news_id)
INNER JOIN tags AS t
ON (tn.tags_id = t.id)
LEFT JOIN
(SELECT COUNT(comments.id) AS count, comments.news_id FROM comments
GROUP BY comments.news_id) AS c
ON n.id = c.news_id
WHERE t.slug = 'nepal'
ORDER BY n.created_date DESC;


10) SELECT TAG CLOUD


SELECT t . * , IFNULL( tq.count, 0 ) AS count
FROM tags AS t
LEFT JOIN (
SELECT COUNT( tags_news.id ) AS count, tags_news.tags_id
FROM tags_news
GROUP BY tags_news.tags_id
) AS tq ON t.id = tq.tags_id
ORDER BY t.name ASC
LIMIT 15;

Hope this helps. And thanks for reading.