how to exclude rows that have duplicates in one field

I have a very simple task, but I cannot find any solution. I have two tables, 'articles' and 'categories'

My article table look like this:

id | cat_id | title | content
1      1      Blah     Content 1
2      1      Blah2    Content 2
3      2      Blah3    Content 3

My categories table look like this:

id | title
1     Category 1
2     Category 2

You see I have 2 articles that have the same cat_id. I do not want with duplicate cat_id field. I cannot use DISTINCT, because I will get all articles, because I want all fields out.

so if i use DISTINCT like this:

SELECT DISTINCT a.id, a.cat_id, a.title, a.content FROM articles AS a

I will get everything out, but I want output like this

id | cat_id | title | content
2      1      Blah2    Content 2
3      2      Blah3    Content 3

Can someone help me please !!!

Solution

This query will select the first article (lowest id) from each category

SELECT a.* FROM Article a LEFT JOIN 
   Article a2 ON a.id<a2.id AND a.cat_id=a2.cat_id
WHERE a2.id IS NULL

It does an outer join with all other articles. The join clause only joins articles with the same category and with a smaller ID. When there are no matches (a2.id is NULL), then we have the article with the lowest ID for that category.