How to group continuous ranges using MySQL

Question

I have a table that contains categories, dates and rates. Each category can have different rates for different dates, one category can have only one rate at a given date.

Answer 1

SELECT catid,min(ddate),max(ddate),rate
FROM (
    SELECT
        Catid,
        Ddate,  
        rate,
        @rn := CASE WHEN (@prev <> rate 
           or DATEDIFF(ddate, @prev_date)>1) THEN @rn+1 ELSE @rn END AS rn,
        @prev := rate,
        @prev_id := catid ,
        @prev_date :=ddate
    FROM (
        SELECT CatID,Ddate,rate 
        FROM rankdate
        ORDER BY CatID, Ddate ) AS a , 
        (SELECT @prev := -1, @rn := 0, @prev_id:=0 ,@prev_date:=-1) AS vars      

) T1 group by catid,rn

Note: The line (SELECT @prev := -1, @rn := 0, @prev_id:=0 ,@prev_date:=-1) AS vars is not necessary in Mysql Workspace, but it is in the PHP mysql_query function.

SQL FIDDLE HERE