Question

Let’s say I have a table named members with this structure:

member_id, col1, col2, number

Now, if I run this query:

SELECT @rownum:=@rownum+1 number, member_id 
FROM members, (SELECT @rownum:=0) r ORDER BY col1 DESC, col2 DESC

I get a number for each row. Is it possible to assign this number to column number for each row?

For example, if the table contained this data:

1 1000 1.2 0
2 8700 1.1 0
3 1000 1.1 0

First row number column should be set to 2, second row to 1 and third row to 3.

Answer 1

You may use a temporary table to store the values and UPDATE from it with a join.

/* Store query result in a temporary table */
CREATE TEMPORARY TABLE ranks AS
  SELECT @rownum:=@rownum+1 number, member_id FROM members, (SELECT @rownum:=0) r ORDER BY col1 DESC, col2 DESC;

/* And join against it to update your members.number column */
UPDATE 
  members
  JOIN ranks ON members.member_id = ranks.member_id
SET members.number = ranks.number;

DROP TABLE ranks;

Actually by my test just now I was able to do this without the temporary table. I didn’t think MySQL would permit updating the same table from a subquery but apparently it is permitted with the correlated subquery.

UPDATE 
  members 
  /* JOIN against a subquery which returns the rank values */
  JOIN ( 
   SELECT @rownum:=@rownum+1 number, member_id 
   FROM members, (SELECT @rownum:=0) r
   ORDER BY col1 DESC, col2 DESC
  ) ranks ON members.member_id = ranks.member_id 
/* And set the number column to the rank */
SET members.number = ranks.number;

SQLfiddle.com demo