联合不同的列在一个然后由此排序

Question

after a simple query...

SELECT Col_ID, datetime1, datetime2, datetime3 
FROM TABLE_DATETIME 
WHERE datetime1 LIKE '20130805%'
    OR datetime2 LIKE '20130805%' 
    OR datetime3 LIKE '20130805%'

... I have this scenario

Col_ID    |datetime1     |datetime2     |datetime3     |
----------|--------------|--------------|--------------|
40302025  |20130805123022|NULL          |NULL          |
40302028  |20130805180055|NULL          |NULL          |
40302030  |NULL          |20130805090055|NULL          |
40302055  |NULL          |20130805190055|NULL          |
40302074  |NULL          |NULL          |20130805070055|

Now, in the same previous query, I want to merge the datetime1, datetime2, datetime3 in one columns which we will call "ALL_DATETIME" then order by this to have this results...

Col_ID    |ALL_DATETIME  |
----------|--------------|
40302074  |20130805070055|
40302030  |20130805090055|
40302025  |20130805123022|
40302028  |20130805180055|
40302055  |20130805190055|

Answer 1

You can use COALESCE() function for this:

SELECT Col_ID
     , COALESCE(datetime1,datetime2,datetime3) AS ALL_DATETIME 
FROM TABLE_DATETIME
WHERE ...
ORDER BY COALESCE(datetime1,datetime2,datetime3)

Output:

|   COL_ID |   ALL_DATETIME |
-----------------------------
| 40302074 | 20130805070055 |
| 40302030 | 20130805090055 |
| 40302025 | 20130805123022 |
| 40302028 | 20130805180055 |
| 40302055 | 20130805190055 |

See this SQLFiddle

Answer 2

Use NULL handling function

SELECT Col_ID, ifnull(datetime1, ifnull(datetime2, datetime3 )) as ALL_DATETIME 
FROM TABLE_DATETIME 
WHERE datetime1 LIKE '20130805%'
OR datetime2 LIKE '20130805%' 
OR datetime3 LIKE '20130805%'
order by 2

Answer 3

try this

SELECT Col_ID, greatest (coalesce(datetime1,0), coalesce(datetime2,0),coalesce( datetime3,0) ) alldatetime
FROM TABLE_DATETIME 
WHERE datetime1 LIKE '20130805%'
OR datetime2 LIKE '20130805%' 
OR datetime3 LIKE '20130805%'

DEMO here