MySQL - Show record pairs in one record

Question

I have the following data in one table:

Time, Type, Kilometers
12:00, 1, 0.1
12:30, 2, 0.2
14:00, 1, 0.4
15:00, 2, 1.0
16:00, 1, 1.2
16:30, 2, 1.5
16:45, 1, 2.0

This data is sorted chronologically using the DateTime field. I would like to show these record 'pairs' as 1 row, like so:

StartTime, Type1Km, Type2Km
12:00, 0.1, 0.2
14:00, 0.4, 1.0
16:00, 1.2, 1.5
16:45, 2.0, NULL

There are a couple of caveats: If there is no Type1 to start, then show NULL in the resulting tables' Type1Km field. Similarly, if there is no Type2 end, show NULL in records' Type2Km field.

How could i do this?

Answer 1

Unfortunately, MySQL lacks a FULL OUTER JOIN, so you'll have to UNION two sets together.

This will get you the cases where Type1Km exists, whether or not Type2Km does.

SELECT
    t1.`Time` as StartTime,
    t1.`Kilometers` as Type1Km,
    t2.`Kilometers` as Type2Km
FROM `times` t1
LEFT JOIN `times` t2 ON t2.`Type` = 2
                    AND t2.`Time` = (SELECT `Time` FROM `times`
                                     WHERE `Time` > t1.`Time`
                                     ORDER BY `Time` LIMIT 1)
WHERE t1.`Type` = 1

Now we need the cases where Type1Km does not exist.

SELECT
    t2.`Time` as StartTime,
    NULL as Type1Km,
    t2.`Kilometers` as Type2Km
FROM `times` t2
LEFT JOIN `times` t1 ON t1.`Time` = (SELECT `Time` FROM `times`
                                     WHERE `Time` < t2.`Time`
                                     ORDER BY `Time` DESC LIMIT 1)
WHERE t2.`Type` = 2
  AND (t1.`Type` = 2 OR t1.`Type` IS NULL)

UNION those together, and you have the desired result:

(
SELECT
    t1.`Time` as StartTime,
    t1.`Kilometers` as Type1Km,
    t2.`Kilometers` as Type2Km
FROM `times` t1
LEFT JOIN `times` t2 ON t2.`Type` = 2
                    AND t2.`Time` = (SELECT `Time` FROM `times`
                                     WHERE `Time` > t1.`Time`
                                     ORDER BY `Time` LIMIT 1)
WHERE t1.`Type` = 1

) UNION ALL (

SELECT
    t2.`Time` as StartTime,
    NULL as Type1Km,
    t2.`Kilometers` as Type2Km
FROM `times` t2
LEFT JOIN `times` t1 ON t1.`Time` = (SELECT `Time` FROM `times`
                                     WHERE `Time` < t2.`Time`
                                     ORDER BY `Time` DESC LIMIT 1)
WHERE t2.`Type` = 2
  AND (t1.`Type` = 2 OR t1.`Type` IS NULL)
)

ORDER BY `StartTime`

---

Update

In my previous query, I forgot to account for having a "type 2" record at the very beginning. Updated to account for that. Here's the results I get:

Data in times table:

+----------+------+------------+
| Time     | Type | Kilometers |
+----------+------+------------+
| 11:00:00 |    2 |        0.1 |
| 12:00:00 |    1 |        0.1 |
| 12:30:00 |    2 |        0.2 |
| 14:00:00 |    1 |        0.4 |
| 14:30:00 |    1 |        0.8 |
| 15:00:00 |    2 |        1.0 |
| 15:30:00 |    2 |        0.2 |
| 16:00:00 |    1 |        1.2 |
| 16:30:00 |    2 |        1.5 |
| 16:45:00 |    1 |        2.0 |
+----------+------+------------+

Results of query:

+-----------+---------+---------+
| StartTime | Type1Km | Type2Km |
+-----------+---------+---------+
| 11:00:00  |    NULL |     0.1 |
| 12:00:00  |     0.1 |     0.2 |
| 14:00:00  |     0.4 |    NULL |
| 14:30:00  |     0.8 |     1.0 |
| 15:30:00  |    NULL |     0.2 |
| 16:00:00  |     1.2 |     1.5 |
| 16:45:00  |     2.0 |    NULL |
+-----------+---------+---------+