MySQL: Conditionally selecting next and previous rows.

Question

http://thedailywtf.com/Articles/The-Hot-Room.aspx

You see how at the bottom there're links to the next and previous articles ("Unprepared For Divide_By_Zero" and "A Completely Different Game")? How do I do that, but selecting the next and previous non-private articles? This works for selecting the next article:

SELECT * FROM articles WHERE id > ? AND private IS NULL

But I cannot find a way to select the previous article.

What is the proper/efficient way to do this, preferably in one query?

Answer 1

Here's how I would do it:

-- next
SELECT * FROM articles WHERE id > ? AND private IS NULL ORDER BY id ASC LIMIT 1

-- previous
SELECT * FROM articles WHERE id < ? AND private IS NULL ORDER BY id DESC LIMIT 1

I'm not sure how to do it in one query. The only thing I can think of is possibly getting both the article you're displaying and the next article in one query, but that might be too confusing.

Answer 2

Or extending Jeremy's answer...
In one query

(SELECT * FROM articles WHERE id > ? 
 AND private IS NULL 
 ORDER BY id ASC LIMIT 1) 
UNION 
(SELECT * FROM articles WHERE id < ? 
 AND private IS NULL 
 ORDER BY id DESC LIMIT 1)

Answer 3

How about a nested select?

SELECT * FROM articles WHERE id IN (
    SELECT id FROM articles WHERE id > ? AND private IS NULL ORDER BY id ASC LIMIT 1)
)
OR id IN (
    SELECT id FROM articles WHERE id < ? AND private IS NULL ORDER BY id DESC LIMIT 1
);

Answer 4

You can get away with subselects etc in your particular case, but if you need anything more complicated (for example: given an initial balance and a list of payments and chargebacks, calculate account balance at every point of time) you probably would want to write a stored procedure that uses SQL REPEAT/WHILE/LOOP clauses and allows use of variables and so on.