Count rows with same value while keeping columns

Lets say i have the following sql table:

id  item_count  texture_count
30  82          205            
30  133         406             
35  253         658             
30  552         1004            
35  234         475             
35  119         288             
30  104         115            
36  594         1422            

How can i modify my select query to return my data with an additional column telling me the number of occurrences for each ID, while keeping the original columns collected? Which would result in this:

id  item_count  texture_count   id_count
30  82          205             4
30  133         406             4
35  253         658             3
30  552         1004            4
35  234         475             3
35  119         288             3
30  104         115             4
36  594         1422            1

Query is this:

SELECT 
    CAST ((2 * FLOOR(idvalue / 2)) AS INT) as id, -- im creating an id based on this operation, currently id value is a float
    item_count,
    texture_count
FROM 
    db

Answer

You can use window functions, if your database supports them. What you ask for is as simple as a window count:

select
    t.*,
    count(*) over(partition by id) id_count
from mytable