Question

To start things off, I want to make it clear that I’m not trying to order by descending order.

I am looking to order by something else, but then filter further by displaying things in a second column only if the value in that column 1 row below it is less than itself. Once It finds that the next column is lower, it stops.

Example:

Ordered by column-------------------Descending Column
353215                                    20
535325                                    15
523532                                    10
666464                                    30
473460                                    20

If given that data, I would like it to only return 20, 15 and 10. Because now that 30 is higher than 10, we don’t care about what’s below it.

I’ve looked everywhere and can’t find a solution.

Answer 1

EDIT: removed the big number init, and edd the counter in ifnull test, so it works in pure MySQL: ifnull(@prec,counter) and not ifnull(@prec,999999).

If your starting table is t1 and the base request was:

select id,counter from t1 order by id;

Then with a mysql variable you can do the job:

SET @prec=NULL;
select * from (
    select id,counter,@prec:= if(
       ifnull(@prec,counter)>=counter,
       counter,
       -1) as prec
    from t1 order by id
 ) t2 where prec<>-1;

except here I need the 99999 as a max value for your column and there’s maybe a way to put the initialisation of @prec to NULL somewhere in the 1st request.

Here the prec column contains the 1st row value counter, and then the counter value of each row if it less than the one from previous row, and -1 when this becomes false.

Update

The outer select can be removed completely if the variable assignment is done in the WHERE clause:

SELECT @prec := NULL;
SELECT
  id,
  counter
FROM t1
WHERE
  (@prec := IF(
    IFNULL(@prec, counter) >= counter,
    counter,
    -1
  )) IS NOT NULL
  AND @prec <> -1
ORDER BY id;

regilero EDIT:
I can remove the 1st initialization query using a temporary table (left join) of 1 row this way: but this may slow down the query, maybe.

(...)
FROM t1 
LEFT JOIN (select @prec:=NULL as nullinit limit 1) as tmp1 ON tmp1.nullinit is null
(..)

As said by @Mike using a simple UNION query or even :

(...)
FROM t1 , (select @prec:=NULL) tmp1 
(...)

is better if you want to avoid the first query.

So at the end the nicest solution is:

SELECT NULL AS id, NULL AS counter FROM dual WHERE (@prec := NULL) 
UNION
SELECT   id,   counter
FROM t1  
WHERE   (
   @prec := IF(
      IFNULL(@prec, counter) >= counter,
      counter,
      -1   )) IS NOT NULL
AND @prec <> -1
ORDER BY id;

+--------+---------+
| id     | counter |
+--------+---------+
| 353215 |      20 |
| 523532 |      10 |
| 535325 |      15 |
+--------+---------+

EXPLAIN SELECT output:

+----+--------------+------------+------+---------------+------+---------+------+------+------------------+
| id | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | Extra            |
+----+--------------+------------+------+---------------+------+---------+------+------+------------------+
|  1 | PRIMARY      | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE | 
|  2 | UNION        | t1         | ALL  | NULL          | NULL | NULL    | NULL |    6 | Using where      | 
| NULL | UNION RESULT | <union1,2> | ALL  | NULL          | NULL | NULL    | NULL | NULL | Using filesort   | 
+----+--------------+------------+------+---------------+------+---------+------+------+------------------+