使用Count with Case表达式

Question

I'm probably going to torn apart for asking but why do these produce different results when comparing Claims_Completed in the 2 different scenarios? The Claims_Completed values are the same as Claims_Received in both scenarios.

SELECT
    DischargeType
   ,COUNT(CASE WHEN (DateReceived > '2/1/2015' AND DateReceived < DATEADD(dd, 1, '2/28/2015')) THEN 1 ELSE 0 END) AS Claims_Received
   ,COUNT(CASE WHEN (DateCompleted > '2/1/2015' AND DateCompleted < DATEADD(dd, 1, '2/28/2015')) THEN 1 ELSE 0 END) AS Claims_Completed 
FROM Claims GROUP BY DischargeType

Scenario 2:

SELECT COUNT(*) AS Claims_Received 
FROM Claims 
WHERE DateReceived > '2/1/2015' AND DateReceived < DATEADD(dd, 1, '2/28/2015') 
GROUP BY DischargeType

SELECT COUNT(*) AS Claims_Completed 
FROM Claims 
WHERE DateCompleted > '2/1/2015' AND DateCompleted < DATEADD(dd, 1, '2/28/2015') 
GROUP BY DischargeType

Answer 1

You need sum() instead of count() :

SELECT DischargeType, 
      SUM(CASE WHEN (DateReceived > '2/1/2015' AND
                     DateReceived < DATEADD(dd, 1, '2/28/2015')) 
               THEN 1 ELSE 0 
          END) AS Claims_Received,
      SUM(CASE WHEN (DateCompleted > '2/1/2015' AND 
                     DateCompleted < DATEADD(dd, 1, '2/28/2015')) 
               THEN 1 ELSE 0 
          END) AS Claims_Completed 
FROM Claims 
GROUP BY DischargeType;

Because count() misinterpret here, as it will count 0 as value or as data.

Answer 2

COUNT will count all non-null values from the sequence. Since 1 and 0 are both non-null it's counting everything. One option is to use NULL instead of 0:

SELECT
    DischargeType
   ,COUNT(CASE WHEN (DateReceived > '2/1/2015' 
               AND DateReceived < DATEADD(dd, 1, '2/28/2015')) 
               THEN 1 ELSE NULL END) AS Claims_Received
   ,COUNT(CASE WHEN (DateCompleted > '2/1/2015' 
               AND DateCompleted < DATEADD(dd, 1, '2/28/2015')) 
               THEN 1 ELSE NULL END) AS Claims_Completed 
FROM Claims 
GROUP BY DischargeType