count items in each category

I've the Following Table Schema for table Items

id | item_id | category_id

I've sample data in this table

id  | item_id | category_id

1   | 1       | 1
2   | 1       | 2
3   | 2       | 开发者_开发知识库1
4   | 2       | 2
5   | 3       | 1
6   | 1       | 1

Requirement: I need to count all those items which are repeating in each category_id. In the above scenario item_id '1' is repeating in category '1'. I need to count the invalid or repeating item_id against each category_id.

I think this should do it for you:

Select item_Id, category_id
From Table
Group By item_Id, category_id
Having Count(*) > 1

SELECT category_id, item_id, COUNT(*) 
FROM table
GROUP BY category_id, item_id
HAVING COUNT(*) > 1

will give you repeating ones. As for 'invalid' you did not define what these are.