Echo SQL data in order of date added using Union All - php

Question

So I am working with a client to implement a similar system as the "badges and privileges system" on StackExchange. Although in her system, she is looking to use points and rewards for her staff. It's the same basic principle. The users are rewarded points for good team work and gain rewards from these points. I thought it would be handy to add the same kind of feature which SE uses to display these in the top nav bar, where it shows your rep and badges in order of the date you have earned either of them. This is my issue, I have found help retrieving the data together from the two separate tables but am not sure how I would display these results in order of date earned? As an example:
User ID #1 has earned 50 points on 18/12/2015 would be in ap_user_points table
User ID #1 has earned 'The Gift Voucher' reward on '17/12/2015'
If I simply:
echo $row8['reward'] . $row8['points_added']
It would echo as:
The Gift Voucher 50
Where I need it in order by date as:
50
The Gift Voucher
If you look at your rep and badge icon in the nav bar you'll see what I'm getting at here, it's a similar system.
<?php
$user_id = $_SESSION['userid'];
$sql8 = "
SELECT r.reward_id,
r.user_id,
r.reward as reward,
r.date_earned as date_earned,
r.badge_desc,
NULL AS points_added,
NULL AS added_for,
NULL AS date_added
FROM ap_user_rewards as r
WHERE r.user_id = '$user_id'
UNION ALL
SELECT NULL,
NULL,
NULL,
NULL,
NULL,
p.points_added AS points_added,
p.added_for AS added_for,
p.date_added AS date_added
FROM ap_user_points as p
WHERE p.user_id = '$user_id' ORDER BY date_earned DESC, date_added DESC;";
$result8 = $conn->query($sql8);
if ($result8->num_rows > 0) {
// output data of each row
while($row8 = $result8->fetch_assoc()) {
////// NOT SURE WHAT TO ECHO HERE?
}
}
?>

Answer 1

Add another column to the result set. In that new column, populate it from both queries... looks like it would be the date_added expression in the first query and the date_earned expression in the second query. When those are in the same column, then ordering is easy. (This also assumes that these expressions are of the same or compatible datatypes, preferably DATE, DATETIME or TIMESTAMP.)
Then you can order by ordinal position, e.g. ORDER BY 2 to order by the second column in the resultset.
SELECT a1
, b1
, NULL
, NULL
, a1 AS sortexpr
FROM ...
UNION ALL
SELECT NULL
, NULL
, x2
, y2
, x2 AS sortexpr
FROM ...
ORDER BY 5 DESC
That's just one possibility. If you can't add an extra column, to line up the expressions from the two queries, then you need a way to discriminate which query is returning the row. I typically include a literal as a discriminator column.
Then you can use implicit-style UNION syntax, wrapping the queries in parens...
( SELECT 'q1' AS `source`
, a1
, b1 AS date_earned
, NULL
, NULL AS date_added
FROM ...
)
UNION ALL
( SELECT 'q2' AS `source`
, NULL
, NULL AS date_earned
, x2
, y2 AS date_added
FROM ...
)
ORDER BY IF(`source`='q1',date_earned,date_added) DESC
Followup
I may have misunderstood the question. I though the question was how to get the rows from a UNION/UNION ALL returned in a particular order.
Personally, I would write the query to include a discriminator column, and then line up the columns as much as I could, so they would be processed the same.
As an example:
SELECT 'reward' AS `source`
, r.date_earned AS `seq`
, r.user_id AS `user_id`
, r.date_earned AS `date_earned`
, r.reward_id
, r.reward
, r.badge_desc
, NULL AS `points_added`
, NULL AS `added_for`
FROM r ...
UNION ALL
SELECT 'points' AS `source`
, p.date_added AS `seq`
, p.user_id AS `user_id`
, p.date_added AS `date_earned`
, NULL
, NULL
, NULL
, p.points_added AS `points_added`
, p.added_for AS `added_for`
FROM p ...
ORDER BY 2 DESC, 1 DESC
(It's probably not really necessary to return user_id, since we already know what the value will be. I've returned it here to demonstrate how the columns from the two resultsets can be "lined up".)
Then, when I fetched the rows...
if ( $row8['source'] == 'points' ) {
# process columns from a row of 'points' type
echo $row8['badge_desc'];
echo $row8['user_id'];
} elsif ( $row8['source'] == 'reward' ) {
# process columns from a row of 'reward' type
echo $row8['added_for'];
echo $row8['user_id'];
}
That's how I would do it.