MySQL - Get a counter for each duplicate value

Question

I have a table with two columns.

+------+------+
| data | num  | 
+------+------+
| a    |      | 
| a    |      |
| a    |      |
| b    |      |
| b    |            


        

Answer 1

Here is a simple query that will do what you want.

select id,data,rownum 
  from (
          select id,
                 data,
                 @row:=if(@prev=data,@row,0) + 1 as rownum,
                 @prev:=data 
            from tbl
        order by data,id
)t

I have included an id on each row. But you don't need it.

Go fiddle: http://sqlfiddle.com/#!2/1d1f3/11/0

Credit: Want Row Number on Group of column in MY SQL?