MySql | Display Latest entries of two columns of two rows into single row

Author - Ishpreet Kaur
27.09.2017
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0 Comments
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Suppose we have two tables : articles and article_comments

CREATE TABLE articles(
`id` INT( 11 ) NOT NULL AUTO_INCREMENT ,
`title` VARCHAR( 200 ) NOT NULL ,
`content` VARCHAR( 500 ) NOT NULL ,
`author_name` VARCHAR( 20 ) DEFAULT 1 NOT NULL ,
`author_email` VARCHAR( 50 ) NOT NULL ,
`created_date` TIMESTAMP NOT NULL DEFAULT '0000-00-00 00:00:00',
`last_modified_time` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE NOW( ) ,
PRIMARY KEY ( `id` )
) ENGINE = INNODB;
CREATE TABLE article_comments(
`id` INT( 11 ) NOT NULL AUTO_INCREMENT ,
`article_id` INT( 11 ) NOT NULL ,
`author_comments` VARCHAR( 200 ) ,
`user_name` VARCHAR( 30 ) ,
`user_email` VARCHAR( 50 ) ,
`user_comments` VARCHAR( 200 ) ,
`created_date` TIMESTAMP NOT NULL DEFAULT '0000-00-00 00:00:00',
PRIMARY KEY ( `id` ) ,
CONSTRAINT `fk_articles` FOREIGN KEY ( `article_id` ) REFERENCES `articles` ( `id` ) ON UPDATE CASCADE ON DELETE CASCADE
) ENGINE = INNODB;

In first table, let we have records:

1

In second table, let we have records:

2

From screenshot 2, it can be observed that, at a time, either author comment is stored or user comment is stored. In this table, article_id is foreign key to articles table. Author_comments column stores comments provided by author of article and user_comments column stores comments provided by any user/public/person worldwide.

Now, suppose we want to fetch records in following form:

3

From screenshot 3, we can see that latest user_comments and author_comments are displaying in single row per article_id. If any of user_comments or author_comments does not exist, then NULL will be displayed. Latest user comments and author comments per article basis, have been fetched in single row, with the following query:

SELECT t.article_id, MAX( t.author_comments ) AS author_comments, MAX( t.user_comments ) AS user_comments
FROM (

SELECT article_id, MAX(
CASE WHEN author_comments IS NOT NULL
THEN id
END ) AS author_comment_id, MAX(
CASE WHEN user_comments IS NOT NULL
THEN id
END ) AS user_comment_id
FROM article_comments
GROUP BY article_id
) AS x
JOIN article_comments AS t ON t.id
IN (
author_comment_id, user_comment_id
)
GROUP BY article_id;

Now, suppose we also want to show author name, title from articles table along with data shown in screenshot 3, then this can be implemented with following query:

SELECT title, author_name, author_comments, user_comments
FROM articles
LEFT JOIN (

SELECT t.article_id, MAX( t.author_comments ) AS author_comments, MAX( t.user_comments ) AS user_comments
FROM (

SELECT article_id, MAX(
CASE WHEN author_comments IS NOT NULL
THEN id
END ) AS author_comment_id, MAX(
CASE WHEN user_comments IS NOT NULL
THEN id
END ) AS user_comment_id
FROM article_comments
GROUP BY article_id
) AS x
JOIN article_comments AS t ON t.id
IN (
author_comment_id, user_comment_id
)
GROUP BY article_id
) AS comments_table ON comments_table.article_id = articles.id;

And finally, we have output as shown below with above query:

4

According to requirements, we can display other fields also like author_email or user_name or user_email etc.

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