Tweets implementation MySQL and queries - mysql

Question

I am implementing a simple follow/followers system in MySQL. So far I have three tables that look like:
CREATE TABLE IF NOT EXISTS `User` (
`user_id` INT AUTO_INCREMENT PRIMARY KEY,
`username` varchar(40) NOT NULL ,
`pswd` varchar(255) NOT NULL,,
`email` varchar(255) NOT NULL ,
`first_name` varchar(40) NOT NULL ,
`last_name` varchar(40) NOT NULL,
CONSTRAINT uc_username_email UNIQUE (username , email)
);
-- Using a middle table for users to follow others on a many-to-many base
CREATE TABLE Following (
follower_id INT(6) NOT NULL,
following_id INT(6) NOT NULL,
KEY (`follower_id`),
KEY (`following_id`)
)
CREATE TABLE IF NOT EXISTS `Tweet` (
`tweet_id` INT AUTO_INCREMENT PRIMARY KEY,
`text` varchar(280) NOT NULL ,
-- I chose varchar vs TEXT as the latter is not stored in the database server’s memory.
-- By querying text data MySQL has to read from it from the disk, much slower in comparison with VARCHAR.
`publication_date` DATETIME NOT NULL,,
`username` varchar(40),
FOREIGN KEY (`username`) REFERENCES `user`(`username`)
ON DELETE CASCADE
);
Lets say I want to write a query that returns the 10 latest tweets by users followed by the user with username "Tom". What is the best way to writhe that query and return results with username, first name, last name, text and publication date.
Also if one minute later I want to query again 10 latest tweets and assuming someone Tom follows tweets during that minute, how do I query the database to not select tweets that have already shown in the first query?

Answer 1

To answer your first question:
SELECT u1.username, u1.first_name, u1.last_name, t.text, t.publication_date
FROM Tweet t
JOIN User u1 ON t.username = u1.username
JOIN Following f ON f.following_id = u1.user_id
JOIN User u2 ON u2.user_id = f.follower_id
WHERE u2.username = 'Tom'
ORDER BY t.publication_date DESC
LIMIT 10
For the second part, simply take the tweet_id from the first row of the first query (so the latest tweet_id value) and use it in the WHERE clause for the next query i.e.
WHERE u2.username = 'Tom'
AND t.tweet_id > <value from previous query>

Answer 2

To get latest 10 tweets for Tom:
select flg.username, flg.first_name, flg.last_name, t.tweet_id, t.text, t.publication_date
from user flr
inner join following f on f.follower_id = flr.user_id
inner join user flg on flg.user_id = f.following_id
inner join tweet t on t.username = flg.username
where flr.username = 'Tom'
order by tweet_id desc
limit 10
To get the next 10 tweets, pass in the max tweet_id, and apply an additional condition in the where clause:
where flr.username = 'Tom'
and t.tweet_id > <previous_max_tweet_id>