通过联接表的特定ID对mysql结果进行排序

Question

I have three tables

Products

id, name, desc

Flags

id, name

Product_flags

Product_id, Flag_id

Some Products have Flags linked to them while others not. I need select products and to give preferences to specific Flags in the select query.

This is my query to give preferences to flag ids 2,3,9, followed by other flags, and followed by non-flagged products ..

SELECT p.*, f.name flagname FROM products p 
LEFT JOIN products_flags pf ON pf.product_id=p.id 
LEFT JOIN flags f ON f.id = pf.flag_id
ORDER BY f.id=2, f.id=3, f.id=9, f.id, p.name

This sort order is not valid. How do I get it right?

Thanks

Answer 1

It's a strange request, but I think you might be able to get around it with a case statement in your order by clause like this:

SELECT 
    p.*, f.name flagname 
FROM products p 
    LEFT JOIN products_flags pf ON pf.product_id=p.id 
    LEFT JOIN flags f ON f.id = pf.flag_id
ORDER BY 
    case
        when f.id=2 then f.id-10
        when f.id=3 then f.id-10
        when f.id=9 then f.id-10
        default f.id
    end,
    p.name

Edit: A column to order by might be a good idea when you want "funky" ordering. As for the case statement, what it does is returns a value to sort by but does so in this manner - sort of like an if statement:

if id=2 then treat it like -8
if id=3 then treat it like -7
if id=3 then treat it like -1
otherwise just use whatever is in id