如何在mysql中为两列选择唯一值

Question

I have created a table

CREATE TABLE `region_details` (
  `id` int(5) NOT NULL,
  `file_id` varchar(20) NOT NULL,
  `region` varchar(30) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

Lets assume I have 10 records as follows

id     file_id     region
1      aaa         a
2      bbb         b
3      ccc         a
4      ddd         c
5      abc         d
6      a01         a
7      j05         b
8      005         c
9      a1021       a
10     111j        b

I need id and region for distinct region values

   id     region
    1     a
    2     b
    4     c
    5     d

Answer 1

Simply do a GROUP BY , use MIN(id) to pick each region's first id.

select min(id) as id, region
from region_details
group by region

Answer 2

You can do it like this:

SELECT min(id),region
FROM YourTable
GROUP BY region

Answer 3

The other two answers are rolling up Id into an aggregate function ( min , or max or some other reduce-function), but looking at the data, it may turn out to be the case that what you want are all the unique combinations of id and region, which you can find by using the following:

select id, region
from yourtable
group by id, region

If you want to get an idea of how/whether/which columns are rolled up, due to the existence of multiple files for a given id/region combination, you might try:

select id, region, count(*)
from yourtable
group by id, region

Any rows with a count(*) value higher than 1 are indications that you've got multiple files scurried away under those particular combinations.