SET @rank:=0;
update T
set Number=@rank:=@rank+1;
UPDATE
alternative way with one statement
UPDATE T
JOIN (SELECT @rank := 0) r
SET Number=@rank:=@rank+1;
SET @rank:=0;
update T
set Number=@rank:=@rank+1;
UPDATE
alternative way with one statement
UPDATE T
JOIN (SELECT @rank := 0) r
SET Number=@rank:=@rank+1;