Question

我尝试在子查询表中计算总员工数。假设计数结果将返回0，但它会一直返回1.

如果我尝试只返回employee_id和month，我没有得到任何正确的返回值，但每次我尝试计数（不同）时，我会得到1作为我的返回值。这是我的sql

SELECT

  count (distinct(CASE WHEN (x.month =5 and x.employee_id <> 0) THEN 
  x.employee_id
        ELSE 0 END)) as test_may
FROM(
 (
SELECT 

     h.month,
     h.employee_id,
     eb.employee_no,
     ee.company_code,
     h.amount,   
     h.year, 
     h.trx_type,   
     h.trx_code, 
     v.trx_desc,
    h.frequency,
    h.run_sequence

FROM 
    v_employee h,
    v_trans v,
    employee_emp ee, 
    employee eb          
  WHERE
        ( h.year = 2014 ) AND  
        ( h.employee_id = ee.employee_id ) AND  
        ( ee.employee_id = eb.employee_id ) AND 
        ( h.employee_no = eb.employee_no ) AND 
        ( h.trx_code = v.trx_code ) AND
        ( h.trx_type = v.trx_type ) AND
        ( v.company_code = ee.company_code OR v.company_code is NULL) AND
        ( h.trx_type IN ('S','B','N','O','A','D','L') )
)
)x,
employee_emp ee,
employee eb

 WHERE

    ( x.employee_id = ee.employee_id ) AND
    ( ee.employee_id = eb.employee_id ) AND
    ( x.employee_no = eb.employee_no ) AND
     ( x.year = 2014 )

Answer 1

现在的count也将计算ELSE表达式的CASE子句中的0。即使使用DISTINCT，仍然会计算该0的一个实例。

删除ELSE 0，以便NULL - 不计算在内：

count (distinct(CASE WHEN x.month =5 and x.employee_id <> 0
                     THEN x.employee_id
                END)) as test_may

请注意，使用NULLIF，您可以将此表达式缩短为：

count (distinct(CASE x.month WHEN 5 THEN NULLIF(x.employee_id, 0) END)) as test_may

Answer 2

即使在这两种情况下，您的count也会返回相同的计数。因为在两种情况下都给出了Count函数的值。

从

改变

 count (distinct(CASE WHEN (x.month =5 and x.employee_id <> 0) THEN 
  x.employee_id
        ELSE 0 END))

要

 count (distinct(CASE WHEN (x.month =5 and x.employee_id <> 0) THEN 
  x.employee_id
        ELSE NULL END))

Count只计算0或100的值为1＆amp;计数时跳过空值。因此，在Else条件NULL将为您提供正确的输出。

COUNT（DISTINCT（））返回false值

2 个答案: