MySQL - 选择所有列，其中一列是 DISTINCT

Question

I'm very sorry if the question seems too basic.
I've surfed entire Internet and StackOverflow for a finished solution, and did not find anything that I can understand, and can't write it myself, so have to ask it here.

I have a MySQL database.
It has a table named "posted".
It has 8 columns.

I need to output this result:

SELECT DISTINCT link FROM posted WHERE ad='$key' ORDER BY day, month

But I need not only the "link" column, but also other columns for this row.
Like for every row returned with this query I also need to know its "id" in the table, "day" and "month" values etc.

Please tell me what should I read to make it, or how to make it.
Please keep it as simple as possible, as I'm not an expert in MySQL.

Edit: I tried this:

SELECT DISTINCT link,id,day,month FROM posted WHERE ad='$key' ORDER BY day, month

It doesn't work. It returns too many rows. Say there are 10 rows with same links, but different day/month/id. This script will return all 10, and I want only the first one (for this link).

Answer 1

The problem comes from instinctively believing that DISTINCT is a local pre-modifier for a column.

Hence, you "should" be able to type

XXbadXX SELECT col1, DISTINCT col2 FROM mytable XXbadXX

and have it return unique values for col 2. Sadly, no. DISTINCT is actually a global post-modifier for SELECT , that is, as opposed to SELECT ALL (returning all answers) it is SELECT DISTINCT (returning all unique answers). So a single DISTINCT acts on ALL the columns that you give it.

This makes it real hard to use DISTINCT on a single column, while getting the other columns, without doing major extremely ugly backflips.

The correct answer is to use a GROUP BY on the columns that you want to have unique answers: SELECT col1, col2 FROM mytable GROUP BY col2 will give you arbitrary unique col2 rows, with their col1 data as well.

Answer 2

I tried this:

 SELECT DISTINCT link,id,day,month FROM posted WHERE ad='$key' ORDER BY day, month

It doesn't work. It returns too many rows. Say there are 10 rows with same links, but different day/month/id. This script will return all 10, and I want only the first one (for this link).

What you're asking doesn't make sense.

Either you want the distinct value of all of link, id, day, month , or you need to find a criterion to choose which of the values of id, day, month you want to use, if you just want at most one distinct value of link .

Otherwise, what you're after is similar to MySQL's hidden columns in GROUP BY / HAVING statements , which is non-standard SQL, and can actually be quite confusing.

You could in fact use a GROUP BY link if it made sense to pick any row for a given link value.

Alternatively, you could use a sub-select to pick the row with the minimal id for a each link value (as described in this answer ):

 SELECT link, id, day, month FROM posted
     WHERE (link, id) IN
           (SELECT link, MIN(id) FROM posted ad='$key' GROUP BY link)

Answer 3

SELECT Id, Link, Day, Month FROM Posted
WHERE Id IN(
   SELECT Min(Id) FROM Posted GROUP BY Link)

Answer 4

  SELECT OTHER_COLUMNS FROM posted WHERE link in (
  SELECT DISTINCT link FROM posted WHERE ad='$key' )
  ORDER BY day, month

Answer 5

If what your asking is to only show rows that have 1 link for them then you can use the following:

SELECT * FROM posted WHERE link NOT IN 
(SELECT link FROM posted GROUP BY link HAVING COUNT(LINK) > 1)

Again this is assuming that you want to cut out anything that has a duplicate link.

Answer 6

I think the best solution would be to do a subquery and then join that to the table. The sub query would return the primary key of the table. Here is an example:

select *
from (
          SELECT row_number() over(partition by link order by day, month) row_id
          , *

          FROM posted 
          WHERE ad='$key' 
          ) x
where x.row_id = 1

What this does is the row_number function puts a numerical sequence partitioned by each distinct link that results in the query.

By taking only those row_numbers that = 1, then you only return 1 row for each link.

The way you change what link gets marked "1" is through the order-by clause in the row_number function.

Hope this helps.

Answer 7

SELECT DISTINCT link,id,day,month FROM posted WHERE ad='$key' ORDER BY day, month

OR

SELECT link,id,day,month FROM posted WHERE ad='$key' ORDER BY day, month

Answer 8

If you want all columns where link is unique:

SELECT * FROM posted WHERE link in
     (SELECT link FROM posted WHERE ad='$key' GROUP BY link);

Answer 9

What you want is the following:

SELECT DISTINCT * FROM posted WHERE ad='$key' GROUP BY link ORDER BY day, month

if there are 4 rows for example where link is the same, it will pick only one (I asume the first one).

Answer 10

I had a similar problem, maybe that help someone, for example - table with 3 columns

SELECT * FROM DataTable WHERE Data_text = 'test' GROUP BY Data_Name ORDER BY Data_Name ASC

or

SELECT Data_Id, Data_Text, Data_Name FROM DataTable WHERE Data_text = 'test' GROUP BY Data_Name ORDER BY Data_Name ASC

Two ways work for me.

Answer 11

In MySQL you can simply use "group by". Below will select ALL, with a DISTINCT "col"

SELECT *
FROM tbl
GROUP BY col

Answer 12

SELECT a.* FROM orders a INNER JOIN (SELECT course,MAX(id) as id FROM orders WHERE admission_id=".$id." GROUP BY course ) AS b ON a.course = b.course AND a.id = b.id

With the Above Query you will get unique records with where condition

Answer 13

Select the datecolumn of month so that u can get only one row per link, eg:

select link, min(datecolumn) from posted WHERE ad='$key' ORDER BY day, month

Good luck............

Or

u if you have date column as timestamp convert the format to date and perform distinct on link so that you can get distinct link values based on date instead datetime