在MySQL中按多列排序

Question

I've four columns sale date 4 , sale date 3 , sale date 2 , sale date in my table which are all sale dates. While displaying in page sale date 4 is given first priority if it is not empty we are displaying it and if it is empty then comes sale date 3 and then sale date 2 and then sale date .

My problem is while sorting the results.

When is run the query with order by

order by 
isnull(`sale date 4`), trim(`sale date 4`) , 
isnull(`sale date 3`), trim(`sale date 3`) , 
isnull(`sale date 2`), trim(`sale date 2`) , 
isnull(`sale date`), trim(`sale date`) asc

I'm getting results as follows

**sale date 4** | **sale date 3**  | **sale date 2** | **sale date**
--------------------------------------------------------------------
2013-01-24      | 2013-01-24       | 2013-01-24      | 2013-01-24    
2013-01-31      | 2013-01-31       | 2013-01-31      | 2012-11-30    
2013-02-01      | 2013-02-01       | 2013-02-01      | 2013-02-01    
2013-03-08      | 2013-03-08       | 2013-03-08      | 2013-03-08    
----NULL----    | ----NULL----     | 2013-02-28      | 2012-11-02    
----NULL----    | ----NULL----     | ----NULL----    | 2013-02-28

And when I display in the view it will be as follows

1. Sale date = 2013-01-24
2. Sale date = 2013-01-31
3. Sale date = 2013-02-01
4. Sale date = 2013-03-08
5. Sale date = 2013-02-28
6. Sale date = 2013-02-28

But what I need is

1. Sale date = 2013-01-24
2. Sale date = 2013-01-31
3. Sale date = 2013-02-01
4. Sale date = 2013-02-28
5. Sale date = 2013-02-28
6. Sale date = 2013-03-08

How can I accomplish this?

Answer 1

isnull(column) simply returns true or false . This results in the first 4 records being the first 4 results ( true > false ) and record 5 and 6 being the last two results. The last two results are then sorted by sale date 4 ( NULL in both columns) up to sale date 1 , but they remain the last two items in the ordered list.

What you want to do is probably something like this:

.. ORDER BY IFNULL(`sale date 4`, IFNULL(`sale date 3`,
            IFNULL(`sale date 2`, `sale date 1`)))

Answer 2

You could order by:

coalesce(`sale date 4`, `sale date 3`, `sale date 2`, `sale date`)

COALESCE returns the first non-NULL value, eg.

coalesce(1,2,3,4)             returns 1
coalesce(null,2,3,4)          returns 2
...
coalesce(null,null,null,null) returns null