Question

I have a MySQL table like

id    |  text   |  category   |   active

I select a random line with

SELECT id, text 
FROM table
WHERE category = [category id] AND active = 1 
ORDER BY RAND()
LIMIT 1

Some times this would return no results (e.g. if there is no active row in a specific category). What I need to do in that case is to return a “default” row.

My question is: what is the most efficient way to do this? Should I just create an identical table but just with the default rows, which I would query if the above query gives no results? Or should I add the default rows in the same table? And how would you query it?

Any suggestion is welcome!

EDIT
A few updates to the question:

1. I am excluding the possibility of generating the default text in PHP, as I want it to be customizable, without having to go and change the code.

2. There will be a default row per category

Answer 1

What is the default value you want? The 1st id in the table?

SELECT id, text
FROM (  SELECT id, categori, active, text, 0 AS prio
        FROM table WHERE category = [category id] AND active = 1 
        UNION ALL
        SELECT id, categori, active, text, 1
        FROM table
        ORDER BY id ASC
        LIMIT 1) AS h
ORDER BY prio, RAND()
LIMIT 1

If you wish for a certain default value, add a ENUM column called default('TRUE', 'FALSE'):

SELECT id, text
FROM (  SELECT id, categori, active, text, 0 AS prio
        FROM table WHERE category = [category id] AND active = 1 AND default = 'FALSE'
        UNION ALL
        SELECT id, categori, active, text, 1
        FROM table
        WHERE category = [category id] AND default = 'TRUE'
        ORDER BY id ASC
        LIMIT 1) AS h
ORDER BY prio, RAND()
LIMIT 1

Easier one:

SELECT id, text
FROM table
WHERE category = [category id] AND (active = 1 OR default = 'TRUE')
ORDER BY (default = 'FALSE') DESC, RAND()
LIMIT 1

Explanation:
Basicly this query fetches all the rows you count on to get, and combine it with a default row. They got a prio value each, which you order them in before doing RAND(). The thing remaining is how you wish to store your default rows.