连接表时，将从Server返回重复数据

Question

我有一张这样的桌子：

Email_tbl

plcid       Ecode
----------- -----------
23          001646
24          001646
25          E004
25          2274
25          2208
25          1868

我有另一张桌子Employee

Ecode   Ename
E004     jaseem
2274     jasir
2208      deepu
1868       rupa
001646     shafeer

我想得到Ename of plcid=25，所以我在这里使用这个查询：

SELECT em.Ename
FROM dbo.Email_tbl e
JOIN dbo.EmployeeMaster_tbl em ON em.Ecode IN (SELECT Ecode
                                               FROM Email_tbl
                                               WHERE plcid = 25)

但我的结果出了问题。我的问题出什么问题了？

Answer 1

SELECT  em.Ename
FROM    dbo.Email_tbl e
        JOIN dbo.EmployeeMaster_tbl em ON em.Ecode = e.ECode
WHERE e.plcid = 25 

Answer 2

你为什么要加入这些桌子？为什么有三个答案“修复”一个不必要的连接？

SELECT em.Ename
FROM dbo.EmployeeMaster_tbl em
WHERE em.Ecode IN (SELECT Ecode
                   FROM Email_tbl
                   WHERE plcid = 25)

Answer 3

    SELECT emp.Ename
    FROM Email_tbl e INNER JOIN Employee emp ON (e.Ecode = emp.Ecode)
    WHERE e.plcid = 25

这将导致多行，因为Employee中的多行具有PLCID = 25的Ecode