Database Agnostic – Using SQL ORDER BY for Complex Criteria

database-agnosticorder-by

Let's say my relation is like so

and I wanted to sort by rank, but A's and B's first. So my relation will look like

At first I tried to just have two relations, where I order both of them by rank, then union the two relations, but they just end up getting jumbled together. I've also tried using cases, but I don't understand how it works. I kept getting errors about mixing characters with integers.

Edit: Forgot to mention that the first relation above is what I end up with after a somewhat complex number of views. For example, I created two views, one which only has A and B classes, then I ordered that by rank. Second view just C classes, ordered that by rank. Then I unioned them, but they just get mixed.

Best Answer

You only need a CASE expression in the ORDER BY, to divide the rows into two groups, so the ones with class 'A' or 'B' are put in first group (1) and the all the others in the second (2):

ORDER BY 
    CASE WHEN class IN ('A', 'B') THEN 1 ELSE 2 END,
    rank ;

Related Solutions

Mysql – Slow complex query with group/order

I can see couple things that should improve your query performance.

1 As you already found out there is absolutely no need to join mentioncache. Using EXISTS seems more natural (or IN as you did, but EXISTS may work better from performance point of view).

2 DATE(m.indexed) BETWEEN "2012-09-16" AND "2012-10-16" can be rewritten to m.indexed between "2012-09-16" AND "2012-10-16 23:59:59", so mysql can use index.

3 urlinfluranks doesn't seem to be used anywhere except in LEFT JOIN, why do you need it?

4 f.foreign_id can be either null or m.id, and this is the only reference to favoureditems table, I'd rather use subquery in this case.

Finally, I think you can get the same results without GROUP BY m.id (as far as I understood , mentions.id a primary key).

SELECT   
m.id, m.title, m.title_text, m.content_text, m.url,m.root_url,m.sub_type,m.indexed,  
CASE 
 WHEN EXISTS 
    (SELECT NULL FROM favoureditems f WHERE f.model = "Mention" 
    AND f.foreign_id = m.id AND f.owner_id = 803) THEN m.id 
END AS f.foreign_id,
, v.foreign_id, v.created, mfs.score,  
Image.id,Image.model,Image.foreign_key, Image.dirname,Image.basename,  
(REPLACE(REPLACE(m.host_url, 'http://www.', ''), 'http://', '')) AS Mention__plain_url  
FROM mentions AS m  

LEFT JOIN 
(
  SELECT id,model,foreign_key,dirname,basename 
  FROM attachments Image  
  WHERE model = 'Mention'
  GROUP BY foreign_key
 )Image  ON (Image.foreign_key = m.id)      

LEFT JOIN 
(
   SELECT v.foreign_id, v.created 
   FROM visiteditems AS v  
   WHERE (v.model = "Mention"  AND v.owner_id = 803)  
    GROUP BY v.foreign_id
)v ON (v.foreign_id = m.id)
LEFT JOIN 
(
   SELECT mention_id,score
   FROM mentionfeedscores mfs  
   WHERE mfs.feed_id = '474737584865424564398208323289092'
   GROUP BY mention_id
)mfs ON (mfs.mention_id = m.id )

WHERE m.indexed BETWEEN "2012-09-16" AND "2012-10-16 23:59:59"  
   AND EXISTS 
  (
     SELECT NULL FROM mentioncache mc  
      WHERE mc.mention_id = m.id AND mc.profile_id = 803  
   )    
ORDER BY m.indexed DESC  
LIMIT 10

Oracle – Why Do an ORDER BY for INSERTs AS SELECT?

It can make sense to insert data in order. There's lots of caveats to this though. If the data isn't frequently updated and if you're using certain types of tables or indexes (e.g. IOT, clustered indexes).

Being in order means that if you're doing a range scan of the ordered column (e.g. BETWEEN x AND Y) then the data is more likely to be in a contiguous set of blocks.

Best Answer

Related Solutions

Mysql – Slow complex query with group/order

Oracle – Why Do an ORDER BY for INSERTs AS SELECT?

Related Question