使用php在sql中找到最新条目的列

Question

Using a php code, I want to know in what column the latest entry is located if I for instance is interested in id 2. Then column 'time4' with the time "2016-03-01 10:01:01" is the newest entry.

id      time1                   time2                   time3                   time4
1       2016-01-01 09:27:24     2016-01-02 10:01:01     2016-01-02 17:05:07     2016-01-01 10:01:01
2       2016-01-02 09:27:24     2016-01-03 10:01:01     2016-02-02 17:05:07     2016-03-01 10:01:01
3       2016-01-02 10:27:24     2016-01-03 11:01:01     2016-02-02 18:05:07     2016-03-01 11:01:01

I tried this

<?php
$User=1;
$result = $con->query("SELECT MAX(TimeToSelect) as Output
FROM (
    SELECT time1 as TimeToSelect,id
    FROM tableTime
    union
    SELECT time2,id
    FROM tableTime
    union
    SELECT time3,id
    FROM tableTime
        union
    SELECT time4,id
    FROM tableTime
    ) as A
  WHERE id = '$User'");
$row = $result->fetch_array(MYSQLI_BOTH);

echo $something=$row[0];
?>

But then it echoes the date "2016-03-01 10:01:01" and not the column name "time4".

Answer 1

If you are only contains 4 columns to compare, you can use the below SQL to get what you want.
You can change the id in where condition to get the latest time is entered for particular 'id'. (Each time each id)

SELECT MAX(TimeToSelect) as Output
FROM (
    SELECT time1 as TimeToSelect,id
    FROM tableTime
    union
    SELECT time2,id
    FROM tableTime
    union
    SELECT time3,id
    FROM tableTime
    union
    SELECT time4,id
    FROM tableTime
    ) as A
  WHERE id = 1

If you want to get the columns name, try below SQL:

SELECT ColumnName
FROM (
    SELECT time1 AS TimeToSelect,id,'time1' AS ColumnName
    FROM tableTime
    UNION
    SELECT time2,id,'time2'
    FROM tableTime
    UNION
    SELECT time3,id,'time3'
    FROM tableTime
    UNION
    SELECT time4,id,'time4' 
    FROM tableTime
    ) AS A 
WHERE id = 1 
AND TimeToSelect = (SELECT MAX(TimeToSelect)
                    FROM (
                         SELECT time1 AS TimeToSelect,id,'time1' AS ColumnName
                         FROM tableTime
                         UNION
                         SELECT time2,id,'time2'
                         FROM tableTime
                         UNION
                         SELECT time3,id,'time3'
                         FROM tableTime
                         UNION
                         SELECT time4,id,'time4' 
                         FROM tableTime
                         ) AS A
                    WHERE id = 1
                    )