sql - MySQL - SELECT all columns WHERE one column is 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 col2. 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.