Question

我有以下SQL查询：

 SELECT v.PassNo, VName, e.eName, d.dtName, v.EntryTime
   FROM Visitorlogo_tbl v
        LEFT JOIN EmployeeMaster_tbl e ON v.empid=e.eId
             JOIN DepartmentMaster_tbl d ON v.Deptid=d.dtId
  WHERE v.EntryTime >= '2014-06-29'

我得到以下输出：

PassNo    VName    eName    dtName    EntryTime
100       jaseem   null     admin     2014-06-29 23:17:47.257
101       deepu    sabu     sales     2014-06-29 24:17:47.257
103       rabeeh   null     IT        2014-06-30 23:17:47.257

因为我想连接 ename 和 dtname ，我写了这样的查询：

 SELECT v.PassNo AS 'Badge Id', VName AS VisitorName,
        e.eName + ' ('+ d.dtName+')' AS 'Name(Department)', v.EntryTime
   FROM Visitorlogo_tbl v
             JOIN DepartmentMaster_tbl d ON v.Deptid = d.dtId
        LEFT JOIN EmployeeMaster_tbl e ON v.empid = e.eId
  WHERE v.EntryTime >= '2014-06-29'

但是，这会导致输出不匹配：

Badge Id  VisitorName    Name(Department)   EntryTime
100       jaseem         null               2014-06-29 23:17:47.257
101       deepu          sabu               2014-06-29 24:17:47.257
103       rabeeh         null               2014-06-30 23:17:47.257

为什么我看到NULL而不是徽章ID 100和103的部门名称？

Answer 1

连接字符串，其中一个字符串NULL将始终产生NULL。例如：SELECT NULL + 'Moo'将导致NULL - 始终。 ISNULL()接受2个参数，当第一个参数解析为NULL时，将返回第二个参数 - BOL

你的意思是这样做吗？

SELECT   [Badge Id]         = v.PassNo
        ,VisitorName        = VName
        ,[Name(Department)] = ISNULL(e.eName, '') + ISNULL(' (' + d.dtName + ')', '')
        ,v.EntryTime 
FROM Visitorlogo_tbl        V
JOIN DepartmentMaster_tbl   D ON v.Deptid=d.dtId
LEFT 
JOIN EmployeeMaster_tbl     E ON v.empid=e.eId
WHERE v.EntryTime >= '2014-06-29'

Answer 2

如果将null与任何内容连接起来，它将返回null。因此，您需要检查null以获得正确的结果。您可以使用以下查询

select v.PassNo as 'Badge Id',VName as VisitorName,ISNULL(e.eName,'') + ISNULL(' ('+ d.dtName+')','') as 'Name(Department)',v.EntryTime from Visitorlogo_tbl v
  join DepartmentMaster_tbl d on v.Deptid=d.dtId
left join EmployeeMaster_tbl e on v.empid=e.eId
where v.EntryTime >='2014-06-29'

Answer 3

SELECT  v.PassNo as 'Badge Id',
        VName as VisitorName,
        ISNULL(e.eName,'') + ISNULL(' ('+ d.dtName+')','') as 'Name(Department)',
        v.EntryTime 
FROM    Visitorlogo_tbl v
JOIN     DepartmentMaster_tbl d on v.Deptid=d.dtId
LEFT JOIN EmployeeMaster_tbl e on v.empid=e.eId
WHERE   v.EntryTime >='2014-06-29'

在连接两列时，会发生输出不匹配

3 个答案: