Question

I have the following sql:

SELECT `id` FROM `exchange` WHERE `photo` = 1 ORDER BY `id` DESC LIMIT 18

How can I only get distinct location values?

For example doing this but it returns an error:

SELECT `id`, distinct(`location`) FROM `exchange` WHERE `photo` = 1 ORDER BY `id` DESC LIMIT 18

Example table:

id    location

1        7
2        8
3        4
4        4
5        2
6        32

Example result (when LIMIT 5):

6,5,4,2,1

Answer 1

DISTINCT works for the whole row, not for the value of each column. Remove the column ID and you will have unique location.

SELECT distinct `location`
FROM `exchange` 
WHERE `photo` = 1 
ORDER BY `id` DESC 
LIMIT 18

UPDATE 1

You can use AGGREGATE FUNCTION like MAX() and grouped them by their ID. Try this again,

SELECT location, MAX(ID) ID
FROM exchange
GROUP BY location