MySQL: how to use COALESCE

Question

Say I have the following table:

TABLE: product
===============================================================================
| product_id | language_id | name           | description                     |
===============================================================================
| 1          | 1           | Widget 1       | Really nice widget. Buy it now! |
-------------------------------------------------------------------------------
| 1          | 2           | Lorem  1       |                                 |
-------------------------------------------------------------------------------

How do I query this such that it tries to give me the name and description where language_id = 2, but fall back to language_id = 1 if the column contains a NULL?

In the above example, I should get Lorem 1 for name and Really nice widget. Buy it now! for description.

Answer 1

How about this?

SET @pid := 1, @lid := 2;
SELECT 
    COALESCE(name,(
        SELECT name
        FROM product
        WHERE product_id = @pid AND description IS NOT NULL
        LIMIT 1
    )) name, 
    COALESCE(description,(
        SELECT description
        FROM product
        WHERE product_id = @pid AND description IS NOT NULL
        LIMIT 1
    )) description
FROM product
WHERE product_id = @pid 
    AND (language_id = @lid 
    OR language_id = 1)
ORDER BY language_id DESC
LIMIT 1;

where:

• @pid: current product id
• @lid: current language id
• Values for name and/or description could be null
• language_id = 2 item could not exist