How to order this specific Inner Joins? - php

Question

Right now I'm creating an online game where I list the last transfers of players.
The table that handles the history of players, has the columns history_join_date and history_end_date.
When history_end_date is filled, it means that player left a club, and when it is like the default (0000-00-00 00:00:00) and history_join_date has some date it means player joined the club (in that date).
Right now, I've the following query:
SELECT
player_id,
player_nickname,
team_id,
team_name,
history_join_date,
history_end_date
FROM
players
INNER JOIN history
ON history.history_user_id = players.player_id
INNER JOIN teams
ON history.history_team_id = teams.team_id
ORDER BY
history_end_date DESC,
history_join_date DESC
LIMIT 7
However, this query returns something like (filtered with PHP above):
(22-Aug-2012 23:05): Folha has left Portuguese Haxball Team.
(22-Aug-2012 00:25): mancini has left United.
(21-Aug-2012 01:29): PatoDaOldSchool has left Reign In Power.
(22-Aug-2012 23:37): Master has joined Born To Win.
(22-Aug-2012 23:28): AceR has joined Born To Win.
(22-Aug-2012 23:08): Nasri has joined Porto Club of Haxball.
(22-Aug-2012 18:53): Lloyd Banks has joined ARRIBA.
PHP Filter:
foreach ($transfers as $transfer) {
//has joined
if($transfer['history_end_date']<$transfer['history_join_date']) {
$type = ' has joined ';
$date = date("d-M-Y H:i", strtotime($transfer['history_join_date']));
} else {
$type = ' has left ';
$date = date("d-M-Y H:i", strtotime($transfer['history_end_date']));
}
As you can see, in the transfers order, the date is not being followed strictly (22-Aug => 21-Aug => 22-Aug).
What am I missing in the SQL?
Regards!

Answer 1

The issue is you are ordering based upon two different values. So your results are ordered first by history_end_date, and when the end dates are equal (i.e. when it is the default value), they are then ordered by history_join_date
(Note that your first results are all ends, and then your subsequent results are all joins, and each subset is properly ordered).
How much control do you have over this data structure? You might be able to restructure the history table such that there is only a single date, and a history type of JOINED or END... You might be able to make a view of joined_date and end_date and sort across that...
From what you have in the question I made up the following DDL & Data:
create table players (
player_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
player_nickname VARCHAR(255) NOT NULL UNIQUE
);
create table teams (
team_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
team_name VARCHAR(255) NOT NULL UNIQUE
);
create table history (
history_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
history_user_id INT NOT NULL, history_team_id INT NOT NULL,
history_join_date DATETIME NOT NULL,
history_end_date DATETIME NOT NULL DEFAULT "0000-00-00 00:00:00"
);
insert into players VALUES
(1,'Folha'),
(2,'mancini'),
(3,'PatoDaOldSchool'),
(4,'Master'),
(5,'AceR'),
(6,'Nasri'),
(7,'Lloyd Banks');
insert into teams VALUES
(1,'Portuguese Haxball Team'),
(2,'United'),
(3,'Reign In Power'),
(4,'Born To Win'),
(5,'Porto Club of Haxball'),
(6,'ARRIBA');
insert into history VALUES
(DEFAULT,1,1,'2012-08-01 00:04','2012-08-22 23:05'),
(DEFAULT,2,2,'2012-08-21 19:04','2012-08-22 00:25'),
(DEFAULT,3,3,'2012-08-19 01:29','2012-08-21 01:29'),
(DEFAULT,4,4,'2012-08-22 23:37',DEFAULT),
(DEFAULT,5,4,'2012-08-22 23:28',DEFAULT),
(DEFAULT,6,5,'2012-08-22 23:08',DEFAULT),
(DEFAULT,7,6,'2012-08-22 18:53',DEFAULT);
SOLUTION ONE - History Event View
This is obviously not the only solution (and you'd have to evaluate options as they suit your needs, but you could create a view in MySQL for your history events and join to it and use it for ordering similar to the following:
create view historyevent (
event_user_id,
event_team_id,
event_date,
event_type
) AS
SELECT
history_user_id,
history_team_id,
history_join_date,
'JOIN'
FROM history
UNION
SELECT
history_user_id,
history_team_id,
history_end_date,
'END'
FROM history
WHERE history_end_date <> "0000-00-00 00:00:00";
Your select then becomes:
SELECT
player_id,
player_nickname,
team_id,
team_name,
event_date,
event_type
FROM players
INNER JOIN historyevent
ON historyevent.event_user_id = players.player_id
INNER JOIN teams
ON historyevent.event_team_id = teams.team_id
ORDER BY
event_date DESC;
Benefit here is you can get both joins and leaves for the same player.
SOLUTION TWO - Pseudo column. use the IF construction to pick one or the other column.
SELECT
player_id,
player_nickname,
team_id,
team_name,
history_join_date,
history_end_date,
IF(history_end_date>history_join_date,history_end_date,history_join_date) as order_date
FROM
players
INNER JOIN history
ON history.history_user_id = players.player_id
INNER JOIN teams
ON history.history_team_id = teams.team_id
ORDER BY
order_date DESC;
Building from #Barmar's answer, you can also use GREATEST() to pick the greatest of the arguments. (MAX() is a grouping function... not actually what you're looking for)

Answer 2

I think what you want is:
ORDER BY MAX(history_join_date, history_end_date)