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Asked: 2026-06-11T21:30:18+00:00

I have five tables in MySQL which I am generating some reports from. I

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I have five tables in MySQL which I am generating some reports from. I will abstract the table names slightly for display purposes. Let’s assume the tables I am working with are as described below:

History  (id, serial, date, time) : Stores historic data, this is the main
                                    column we query.
Serial   (id, serial)             : Stores serial numbers used in the next
                                    3 tables.
Employee (id, serial_id, name)
Event    (id, serial_id, name)
Location (id, serial_id, name)

EDIT: Rewrote the whole question in hopes that it is clearer and easier to understand…

I need to query for events, also if an employee serial is found in history directly PRIOR to a row with a serial matching an event, that employee will be associated with that event, likewise, if a location is found in the history directly AFTER the row with a serial matching an event, that location will be associated with that event.

Sample data:

History:
+------+------------+------------+----------+
| id   | serial     | date       | time     |
+------+------------+------------+----------+
| 1    | 00001234   | 2012-09-25 | 09:00:00 |
+------+------------+------------+----------+
| 2    | 00001235   | 2012-09-25 | 09:01:00 |
+------+------------+------------+----------+
| 3    | 00001236   | 2012-09-25 | 09:02:00 |
+------+------------+------------+----------+
| 4    | 00001235   | 2012-09-25 | 09:05:00 |
+------+------------+------------+----------+
| 5    | 00001236   | 2012-09-25 | 09:07:00 |
+------+------------+------------+----------+
| 6    | 00001235   | 2012-09-25 | 09:10:00 |
+------+------------+------------+----------+
| 7    | 00001235   | 2012-09-25 | 09:11:00 |
+------+------------+------------+----------+
| 8    | 00001235   | 2012-09-25 | 09:12:00 |
+------+------------+------------+----------+
| 9    | 00001236   | 2012-09-25 | 09:15:00 |
+------+------------+------------+----------+
| 10   | 00001234   | 2012-09-25 | 09:20:00 |
+------+------------+------------+----------+
| 11   | 00001236   | 2012-09-25 | 09:25:00 |
+------+------------+------------+----------+

Serial:
+----------+------------+
| id       | serial     |
+----------+------------+
| 1        | 00001234   |
+----------+------------+
| 2        | 00001235   |
+----------+------------+
| 3        | 00001236   |
+----------+------------+

Employee:
+----------+------------+-------------+
| id       | serial_id  | name        |
+----------+------------+-------------+
| 1        | 1          | John Smith  |
+----------+------------+-------------+

Event:
+----------+------------+-------------+
| id       | serial_id  | name        |
+----------+------------+-------------+
| 1        | 2          | Event 1     |
+----------+------------+-------------+

Location:
+----------+------------+-------------+
| id       | serial_id  | name        |
+----------+------------+-------------+
| 1        | 3          | Location 1  |
+----------+------------+-------------+

If I were to do a normal LEFT JOIN query (just for clarity as the difference in results), with the following query, results would be:

SELECT
    history.id,
    history.serial,
    history.date,
    history.time,
    employee.name as 'employee',
    event.name as 'event',
    location.name as 'location'
FROM history
LEFT JOIN serial ON history.serial = serial.serial
LEFT JOIN employee ON serial.id = employee.serial_id
LEFT JOIN event ON serial.id = event.serial_id
LEFT JOIN location ON location.id = location.serial_id
ORDER BY date, time

Results on normal LEFT JOIN are (for clarity only, I don’t need this result set):

+----+----------+------------+----------+------------+----------+------------+
| id | serial   | date       | time     | employee   | event    | location   |
+----+----------+------------+----------+------------+----------+------------+
| 1  | 00001234 | 2012-09-25 | 09:00:00 | John Smith | NULL     | NULL       |
+----+----------+------------+----------+------------+----------+------------+
| 2  | 00001235 | 2012-09-25 | 09:01:00 | NULL       | Event 1  | NULL       |
+----+----------+------------+----------+------------+----------+------------+
| 3  | 00001236 | 2012-09-25 | 09:02:00 | NULL       | NULL     | Location 1 |
+----+----------+------------+----------+------------+----------+------------+
| 4  | 00001235 | 2012-09-25 | 09:05:00 | NULL       | Event 1  | NULL       |
+----+----------+------------+----------+------------+----------+------------+
| 5  | 00001236 | 2012-09-25 | 09:07:00 | NULL       | NULL     | Location 1 |
+----+----------+------------+----------+------------+----------+------------+
| 6  | 00001235 | 2012-09-25 | 09:10:00 | NULL       | Event 1  | NULL       |
+----+----------+------------+----------+------------+----------+------------+
| 7  | 00001235 | 2012-09-25 | 09:11:00 | NULL       | Event 1  | NULL       |
+----+----------+------------+----------+------------+----------+------------+
| 8  | 00001235 | 2012-09-25 | 09:12:00 | NULL       | Event 1  | NULL       |
+----+----------+------------+----------+------------+----------+------------+
| 9  | 00001236 | 2012-09-25 | 09:15:00 | NULL       | NULL     | Location 1 |
+-----+---------+------------+----------+------------+----------+------------+
| 10 | 00001234 | 2012-09-25 | 09:20:00 | John Smith | NULL     | NULL       |
+----+----------+------------+----------+------------+----------+------------+
| 11 | 00001236 | 2012-09-25 | 09:25:00 | NULL       | NULL     | Location 1 |
+----+----------+------------+----------+------------+----------+------------+

The query should be based off the event, event should never be NULL. It should look at the row PRIOR and label it as employee (or null if no match), it should look at the row AFTER and label it as location (or null if no match). Example of what the result should be, using the above data:

Result should be:
+----------+------------+------------+------------+----------+------------+
| e_serial | date       | time       | employee   | event    | location   |
+----------+------------+------------+------------+----------+------------+
| 00001235 | 2012-09-25 | 09:01:00   | John Smith | Event 1  | Location 1 |
+----------+------------+------------+------------+----------+------------+
| 00001235 | 2012-09-25 | 09:05:00   | NULL       | Event 1  | Location 1 |
+----------+------------+------------+------------+----------+------------+
| 00001235 | 2012-09-25 | 09:10:00   | NULL       | Event 1  | NULL       |
+----------+------------+------------+------------+----------+------------+
| 00001235 | 2012-09-25 | 09:11:00   | NULL       | Event 1  | NULL       |
+----------+------------+------------+------------+----------+------------+
| 00001235 | 2012-09-25 | 09:12:00   | NULL       | Event 1  | Location 1 |
+----------+------------+------------+------------+----------+------------+

I am not sure if this is possible with some set of sub selects or something, I think this is way over my league with SQL. Currently I have this working with application logic, but thought it might be cleaner and faster to handle it with SQL only.

Any help would be fantastic.

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1 Answer

  1. Editorial Team

    In case someone else ever runs into the issue, posting the solution here. Thanks to dianuj for steering me in the right direction for it.

    SELECT
    serial.serial as 'e_serial',
    @dt:=TIMESTAMP(history.date, history.time) as 'date_time',
    history.date,
    history.time,
    (SELECT
        employee.name
    FROM
        history
        LEFT JOIN serial ON history.serial = serial.serial
        LEFT JOIN employee ON employee.serial_id = serial.id
    WHERE
        TIMESTAMP(history.date, history.time) = (
            SELECT
                max(TIMESTAMP(history.date, history.time))
            FROM
                history
            WHERE TIMESTAMP(history.date, history.time) < @dt
        )
    ) as 'employee',
    event.name as 'event',
    (SELECT
        location.name
    FROM
        history
        LEFT JOIN serial ON history.serial = serial.serial
        LEFT JOIN location ON location.serial_id = serial.id
    WHERE
        TIMESTAMP(history.date, history.time) > @dt
    LIMIT 1
    ) as 'location'
FROM history
LEFT JOIN serial ON history.serial = serial.serial
LEFT JOIN event ON serial.id = event.serial_id
WHERE event.name IS NOT NULL
ORDER BY history.date, history.time

    And the results

    +----------+---------------------+------------+----------+------------+---------+------------+
| e_serial | date_time           | date       | time     | employee   | event   | location   |
+----------+---------------------+------------+----------+------------+---------+------------+
| 00001235 | 2012-09-25 09:01:00 | 2012-09-25 | 09:01:00 | John Smith | Event 1 | Location 1 |
| 00001235 | 2012-09-25 09:05:00 | 2012-09-25 | 09:05:00 | NULL       | Event 1 | Location 1 |
| 00001235 | 2012-09-25 09:10:00 | 2012-09-25 | 09:10:00 | NULL       | Event 1 | NULL       |
| 00001235 | 2012-09-25 09:11:00 | 2012-09-25 | 09:11:00 | NULL       | Event 1 | NULL       |
| 00001235 | 2012-09-25 09:12:00 | 2012-09-25 | 09:12:00 | NULL       | Event 1 | Location 1 |
+----------+---------------------+------------+----------+------------+---------+------------+
5 rows in set (0.01 sec)
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