使用php在sql中找到最新条目的列

Question

使用 php 代码，如果我对 id 2 感兴趣，我想知道最新条目位于哪一列。然后时间为“2016-03-01 10:01:01”的“time4”列是最新的入口。

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

我试过这个

<?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];
?>

但随后它与日期“2016-03-01 10:01:01”相呼应，而不是列名“time4”。

Answer 1

如果你只包含 4 列进行比较，你可以使用下面的 SQL 来获得你想要的。
您可以在 where 条件中更改 id 以获取为特定“id”输入的最新时间。 （每次每个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

如果要获取列名，请尝试以下 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
                    )