“ORDER BY 'column_name' DESC”, but numbers before letters requires two statements?

问题

I want to select data in my database and order it alphabetically but numbers first and then letters.

Sample of my table:

                    watch_date
474 1442437 2181257 2015-12-20 Modern Family   3 13 Little Bo Bleep
475 1442437 2189485 2015-12-21 Modern Family   3 14 Me? Jealous?
476 1442437 2209159 2015-12-22 Modern Family   3 15 Aunt Mommy
477 1442437 2237415 2015-12-22 Modern Family   3 16 Virgin Territory
478 1442437 2244103 2015-12-22 Modern Family   3 17 Leap Day
479 1442437 2239817 2015-12-24 Modern Family   3 18 Send Out the Clowns
480 1442437 2305713 2015-12-24 Modern Family   3 19 Election Day
481 1442437 2305715 2016-01-02 Modern Family   3 20 The Last Walt
482 1442437 2247703 2016-01-03 Modern Family   3 21 Planes, Trains and Cars
483 1553656 2656658 unknown    Under the Dome  1  1 Pilot
484 1553656 2779640 unknown    Under the Dome  1  2 The Fire
485 1553656 2821206 unknown    Under the Dome  1  3 Manhunt

I want to order it using the fourth column (the column with dates and the text 'unknown' (it's called 'watch_date'))

The data in this column is either the text unknown or a date with the format YYYY-MM-DD

To do this I use the following query:

SELECT * FROM `watched_episodes` ORDER BY `watch_date` + 0 DESC, `watch_date` DESC

I thought the + 0 part added a 0 to all results, yet when I do not add the second ORDER BY statement:

`watch_date` DESC

it does put the numbers (dates) before the letters but it does not seem to sort the dates at all.

回答1:

Hmmm, your query is simply wrong. It should be giving an error because of the use of single quotes. I'll assume you intend:

SELECT *
FROM watched_episodes
ORDER BY watch_date + 0 DESC, watch_date DESC;

This seems like a strange method. If the values is always 'Unknown' or a string in YYYY-MM-DD format, I would go for:

ORDER BY (watch_date = 'unknown'),  -- USE `DESC` to put `unknown` first
         watch_date DESC

回答2:

Use "union"

SELECT * FROM (
SELECT * FROM `watched_episodes` WHERE `watch_date` != 'unknown'
ORDER BY `watch_date` DESC
) table_alias
UNION SELECT * FROM `watched_episodes` WHERE `watch_date` = 'unknown';

You can order by whatever you want in subquery and still have the "unknown" rows after the actual date

Tip use "date" as column type of date format - so you can use date function without converting and just let the unknown be null.

来源：https://stackoverflow.com/questions/37102351/order-by-column-name-desc-but-numbers-before-letters-requires-two-statement

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sorting

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