Question

Please help me to sort my table in MySQL. I need to make it only once, not for use in scripts or somewhere else dynamically.

Here’s my table ‘page’:

|  ...  |  title  |  ...  |  parent_id  |  ...  |  position  |  ...  |

I need to update table with the rule: select all rows with the same parent_id, sort them by title DESC and set first row position = 0, second = 1 etc. And loop this for all parent_id.

I made a query to display what I need, but with no UPDATE and only for one parent_id. But as there are some thousands parent_id this query needs a loop. And after selecting, position must be updated to have the same value as @number for current row.

SET @number = -1;
SELECT @number:=@number+1 AS number, p.* FROM `page` AS p WHERE parent_id=1
  ORDER BY title DESC;

Thanks for your help!

Answer 1

SELECT @p:=0, @parent:=0;
UPDATE page p1
JOIN
( SELECT title, 
  CASE WHEN @parent<>parent_id THEN @p:=0 ELSE @p:=@p+1 END as position,
  @parent:=parent_id as parent_id
  FROM page
  ORDER BY parent_id, title DESC ) p2
ON p1.title = p2.title AND p1.parent_id = p2.parent_id
SET p1.position = p2.position

If your (parent_id,title) pairs aren’t unique use your primary key as join condition – you will need to add it to select in parenthesis.