Mysql，在多列索引中的特定行之前获取行

Question

Say I have this table for high-scores:

id : primary key
username : string
score : int

User names and scores themselves can be repeating, only id is unique for each person. I also have an index to get high-scores fast:

UNIQUE scores ( score, username, id )

How can I get rows below the given person? By 'below' I mean they go before the given row in this index.

Eg for ( 77, 'name7', 70 ) in format ( score, username, id ) I want to retrieve:

77, 'name7', 41
77, 'name5', 77
77, 'name5', 21
50, 'name9', 99

but not

77, 'name8', 88 or
77, 'name7', 82 or
80, 'name2', 34 ...

Answer 1

Here's one way to get the result:

SELECT t.score
     , t.username
     , t.id
  FROM scores t
 WHERE ( t.score < 77 ) 
    OR ( t.score = 77 AND t.username < 'name7' )
    OR ( t.score = 77 AND t.username = 'name7' AND t.id < 70 )
 ORDER
    BY t.score DESC
     , t.username DESC
     , t.id DESC

(NOTE: the ORDER BY clause may help MySQL decide to use the index to avoid a " Using filesort " operation. Your index is a "covering" index for the query, so we'd expect to see " Using index " in the EXPLAIN output.)

---

I ran a quick test, and in my environment, this does perform a range scan of the index and avoids a sort operation.

EXPLAIN OUTPUT

id  select_type table type  possible_keys      key        rows Extra                     
--  ----------- ----- ----- ------------------ ---------- ---- --------------------------
 1  SIMPLE      t     range PRIMARY,scores_UX1 scores_UX1    3 Using where; Using index 

---

(You may want to add a LIMIT n to that query, if you don't need to return ALL the rows that satisfy the criteria.)

If you have an unique id of a row, you could avoid specifying the values in the table by doing a join. Given the data in your question:

Here we use a second reference to the same table, to get the row id=70, and then a join to get all the rows "lower".

SELECT t.score
     , t.username
     , t.id
  FROM scores k
  JOIN scores t
    ON ( t.score < k.score ) 
    OR ( t.score = k.score AND t.username < k.username )
    OR ( t.score = k.score AND t.username = k.username AND t.id < k.id )
 WHERE k.id = 70
 ORDER
    BY t.score DESC
     , t.username DESC
     , t.id DESC
 LIMIT 1000

The EXPLAIN for this query also shows MySQL using the covering index and avoiding a sort operation:

id select_type table type  possible_keys      key         rows Extra
-- ----------- ----- ----- ------------------ ----------  ---- ------------------------
 1 SIMPLE      k     const PRIMARY,scores_UX1 PRIMARY       1
 1 SIMPLE      t     range PRIMARY,scores_UX1 scores_UX1    3  Using where; Using index

Answer 2

The concept of "below" for repeating scores is quite fuzzy: Think of 11 users having the same score, but you want the "10 below" a special row. That said, you can do something like (assuming you start with id=70)

SELECT score, username, id 
FROM scores
WHERE score<=(SELECT score FROM scores WHERE id=77)
ORDER BY if(id=77,0,1), score DESC
  -- you might also want e.g. username 
LIMIT 5 -- you might want such a thing
;

Which will give you the rows in question inside this fuzzy factor, with the anchor row first.

Edit

Re-reading your question, you don't want the anchor row, so you need WHERE score<=(...) AND id<>77 and forget the first part of the ORDER BY

Edit 2

After your update to the question, I understand you want only those rows, that have one of

• score < score in anchor row
• score == score in anchor row AND name < name in anchor row
• score == score in anchor row AND name == name in anchor row AND id < id in anchor row

We just have to put that into a query (again assuming your anchor row has id=70):

SELECT score, username, id 
FROM scores, (
  SELECT 
    @ascore:=score, 
    @ausername:=username,
    @aid:=id
  FROM scores 
  WHERE id=70
) AS seed
WHERE
  score<@ascore
  OR (score=@ascore AND username<@ausername)
  OR (score=@ascore AND username=@ausername AND id<@aid)
ORDER BY
  score DESC,
  username DESC,
  id DESC
-- limit 5 //You might want that
;

Answer 3

I think this is the query you want:

select s.*
from scores s
where s.score <= (select score
                  from scores
                  where id = 70
                 ) and
      s.id <> 70
order by scores desc
limit 4;