sql - 如果添加 WHERE 子句，LEFT JOIN 不返回任何结果？

Question

DECLARE @ps_StartDate DATETIME = '20190101'
      , @ps_EndDate   DATETIME = '20191224'
      , @ps_COMPANYNO VARCHAR(20) = '01'

SET @ps_EndDate = DATEADD(dd, 1, @ps_EndDate)

SELECT
    CASE WHEN @ps_COMPANYNO IN ('01') THEN 'YS45J146'
        ELSE 'YS45J147'
    END                                 AS 'Client ID'
    ,e.EMPNO                            AS 'WORKER ID'
    ,''                                 AS 'Org'
    ,''                                 AS 'Job Number'
    ,'Regular'                          AS 'Pay Component'
    ,''                                 AS 'Rate'
    ,''                                 AS 'Rate Number'
    ,SUM(t.TIMEBILLED + (ISNULL(ehrs.AllocationDays, 0) * 24))                  AS 'Hours'
    ,''                                 AS 'Units'
    ,''                                 AS 'Line Date'
    ,''                                 AS 'Amount'
    ,''                                 AS 'Check Seq Number'
    ,''                                 AS 'Override State'
    ,''                                 AS 'Override Local'
    ,''                                 AS 'Override Local Jurisdiction'
    ,''                                 AS 'Labor Assignment'
FROM TIMEBILL t (NOLOCK)
INNER JOIN EMPLOYEE e (NOLOCK) ON t.USERID = e.USERID
LEFT JOIN dbo.EmpHRSchedule ehrs (NOLOCK) ON (e.EmployeeNo = ehrs.EmployeeNo)
WHERE t.PRTID = 'P'
    AND t.DATE >= @ps_StartDate
    AND t.DATE < @ps_EndDate
    AND ehrs.STARTDATE >= @ps_StartDate
    AND ehrs.STARTDATE < @ps_EndDate
    AND e.COMPANYNO = @ps_COMPANYNO
    AND t.SUBCAT <> 'LUNCH'
    AND t.WAGETYPE <> 'OVERTIME'
    AND e.SALARYUNIT = 'HOURLY'
    --AND ehrs.Type = 'HOLIDAY'
GROUP BY e.EMPNO

仅当 dbo.EmpHRSchedule.Type = 'HOLIDAY' 条目存在时，我才尝试将其添加到“小时”列中。据我了解，LEFT JOIN 仅在匹配时才返回条目，对于不匹配则返回 NULL。当我取消注释 AND ehrs.Type = 'HOLIDAY' 行时，根本不返回任何结果。

为什么会出现这样的情况呢？我是否应该采用另一种方法来对 dbo.Timebill.Timebilled 和 dbo.EmpHRSchedule.AllocationDays 进行求和(如果存在)？

Answer 1

当您包括:

AND ehrs.Type = 'HOLIDAY'

在 WHERE 子句中，您实际上得到的是 INNER 连接，而不是 LEFT 连接，因为您只从表中获取匹配的行ers，而不是 ehrs.Type 为 NULL 的任何不匹配行。
您可以做的是将条件移至 LEFT 连接的 ON 子句:

LEFT JOIN dbo.EmpHRSchedule ehrs (NOLOCK) 
ON (e.EmployeeNo = ehrs.EmployeeNo) AND ehrs.Type = 'HOLIDAY' 

您还可以在同一个表中包含其他条件:

LEFT JOIN dbo.EmpHRSchedule ehrs (NOLOCK) 
ON (e.EmployeeNo = ehrs.EmployeeNo) AND ehrs.Type = 'HOLIDAY' 
   AND ehrs.STARTDATE >= @ps_StartDate
   AND ehrs.STARTDATE < @ps_EndDate