使用CASE WHEN的条件计数

Question

I have table like below

CID ITEM    FLAG
C_1 ITEM_1  0
C_1 ITEM_2  1
C_1 ITEM_2  1
C_1 ITEM_2  0
C_2 ITEM_2  0
C_3 ITEM_2  1
C_3 ITEM_2  1
C_3 ITEM_3  1

and I want to produce table like

CID UNIQ_CNT_OF_CID
ITEM_1  0
ITEM_2  2
ITEM_3  1

The catch is - I want to count CID once for same CID .

I tried ,

SELECT  ITEM, CID,
      case when FLAG =1 then count(distinct CID)  else 0 end  as COUNTER 
      from T  
      group by ITEM

Please help !!

Answer 1

I think I understand your question correctly, if not let me know.

You can use a windowed function to accomplish this:

SELECT
ITEM,
COUNT(DISTINCT CID) OVER (PARTITION BY ITEM) AS UNIQ_CNT_OF_CID
FROM T

This will give you the same number of rows, so you if you want the unique from this, you can use a CTE or sub-query it like so:

SELECT DISTINCT
ITEM, UNIQ_CNT_OF_CID
FROM
    (
    SELECT
    ITEM,
    COUNT(DISTINCT CID) OVER (PARTITION BY ITEM) AS UNIQ_CNT_OF_CID
    FROM T
    ) final

Answer 2

You were close. 2 items to point out:

1. The group by must contain the same fields in the select (that are not in an aggregate function). since there is no need to group by CID , that should be removed from the select.
2. You can use CASE for fields not in a group by, but inside the aggregate function.

So amending your SQL will now look like this:

SELECT ITEM,
    -- The CID is used inside the case, inside the count function.
    -- NULL will not be included in the count
    COUNT(DISTINCT CASE WHEN FLAG = 1 THEN CID ELSE NULL END) AS COUNTER 
    from T 
    group by ITEM

Result: