Question

I have a somewhat complicated question related to MySQL. This is the table I have:

keyword   args     title    namespace_id      ratio
en        1        A        23                0.5
en        1        B        89                0.6
en        0        C        89                0.4
foo       1        Foo      23                0.7
bar       1        Bar      89                0.3

I want a list of all rows without duplicates of (keywords,args). If there are duplicates, the picked row should be selected by an order of namespace_ids I provide. The rows left should be ordered by ratio.

Example result with namespace_id order 23,89,x,y:

keyword   args     title    namespace_id      ratio
foo       1        Foo      23                0.7
en        1        A        23                0.5
en        0        C        89                0.4
bar       1        Bar      89                0.3

Example result with namespace order 89,23,x,y:

keyword   args     title    namespace_id      ratio
foo       1        Foo      23                0.7
en        1        B        89                0.6
en        0        C        89                0.4
bar       1        Bar      89                0.3

Is there any way I can to this directly in MySQL? I had a look at GROUP BY, ORDER BY and noticed even the GROUP_CONCAT() function but I didn’t manage to put it all together properly. The statement that does somehow I want is:

  SELECT keyword, args, title, namespace_id, ratio 
    FROM tbl 
GROUP BY keyword, args 
ORDER BY ratio DESC;

But now I don’t know how to bring in the namespace_id order.

I’ve found similar questions like this:
MySQL: "order by" inside of "group by"
The answer there close to what I want, however, my namespace_id order is varying and cannot be calculated by the MAX() function.

EDIT: The challenge here is to tell GROUP BY which row to pick. The normal ORDER BY apparently doesn’t, it only uses the output of GROUP BY.

Answer 1

Try using FIELD() and a subquery:

SELECT t1.keyword, t1.args, t1.title, t1.namespace_id, t1.ratio
FROM tbl t1, (SELECT keyword, args, MIN(FIELD(namespace_id, 23, 89))
minfield FROM tbl GROUP BY keyword, args) t2
WHERE t1.keyword = t2.keyword AND t1.args = t2.args AND
FIELD(t1.namespace_id, 23, 89) = t2.minfield
ORDER BY ratio DESC;

or a self-outer-join:

SELECT t1.keyword, t1.args, t1.title, t1.namespace_id, t1.ratio
FROM tbl t1
LEFT OUTER JOIN tbl t2 ON
    t1.keyword = t2.keyword AND
    t1.args = t2.args AND
    FIELD(t1.namespace_id, 23, 89) < FIELD(t2.namespace_id, 23, 89)
WHERE t2.keyword IS NULL AND t2.args IS NULL
ORDER BY t1.ratio DESC;

Edit: It might also be worth having a look at some (commercial) DBMS supporting OLAP operations (if you’re able to choose and if you are processing large sets of data). For Georg’s case I think the OVER and PARTITION BY keywords would have helped: http://publib.boulder.ibm.com/infocenter/rbhelp/v6r3/topic/com.ibm.redbrick.doc6.3/sqlrg/sqlrg36.htm#sii06377181