MySQL选择if else并按语句排序

Question

Without any training (and deaf) I developed a town portal system but I gave up on a specific select statement.

SELECT `id` FROM `test` ORDER BY FIND_IN_SET(`townid`,'townA'), date DESC LIMIT 3 ;

CREATE TABLE `test` (
  `id` int(5) NOT NULL AUTO_INCREMENT,
  `townid` varchar(10) NOT NULL,
  `data` text NOT NULL,
  `date` date NOT NULL DEFAULT '2017-08-30'
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `test` (`id`, `townid`, `data`, `date`) VALUES
(1, 'townB', 'Data for Town B', '2017-08-29'),
(2, 'townA', 'Data for Town A', '2017-08-28'),
(3, 'townA', 'Data for Town A', '2017-08-27'),
(4, 'townA', 'Data for Town A', '2017-08-26'),
(5, 'townC', 'Data for Town C', '2017-08-25'),
(6, 'townB', 'Data for Town B', '2017-08-24');

ALTER TABLE `test`
  ADD PRIMARY KEY (`id`);

ALTER TABLE `test`
  MODIFY `id` int(5) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=7;COMMIT;

The goal is that

• townA select ID 2, 3 and 4
• townB select ID 1, 6 and 2
• townC select ID 5, 1 and 2

In other words the town select his OWN articles first in date order with limit 3, ELSE select the limit 3 or remaining of the limit 3 in date order from any other town.

Neither select WHERE townid = 'townB' or WHERE townid IN ('townB') can deal with the 3 limit problem where I need THREE results even if there is only 1 of 2 entries for Town B

Answer 1

You were almost perfect in your sample provided and your query. The only thing you need to change is the order by find-in-set clause to DESCENDING.

ORDER BY FIND_IN_SET(townid,'townA') DESC, date DESC

The find_in_set() will return a 1 or 0 based on the thing being found (1) or not (0). So you are ordering by those NOT found first, THEN found. You want the reverse. So, even if you have 99 items, and evenly split over each town A, B and C over a span of several days, having the "DESC"ending order on the FIND_IN_SET will say give me all of 'townA' records first, then anyone else after that. THEN, within each set of FOUND (or NOT) records, sort them in descending date order.

Your query is actually just the one "DESC" clause away.

Answer 2

SELECT * 
  FROM test 
 ORDER 
    BY townid = 'townc' DESC -- or 'towna' or 'townb'
     , date DESC
 LIMIT 3;