Question

How can I join 2 different tables on 2 columns where one column is equal and the other is not. In the example below, I need to query for records where the email matches but the NID does not.

Table A
ID              NID                  Email
1               bob                  [e-mail]
2               mike                 [e-mail]
3               dave                 [e-mail]

Table B
ID              NID                  Email
1               bob                  [e-mail]
2               mike                 [e-mail]
3               doug                 [e-mail]

Sample output should be from Table B:

ID              NID                  Email

3               doug                 [e-mail]

Bonus points if you can do it in LINQ to Entities

Answer 1

CREATE TABLE #TableA
(
    ID int,
    NID varchar(25),
    Email varchar(50)
)

CREATE TABLE #TableB
(
    ID int,
    NID varchar(25),
    Email varchar(50)
)

INSERT INTO #TableA(ID, NID, Email) VALUES (1, 'bob', '[e-mail]')
INSERT INTO #TableA(ID, NID, Email) VALUES (2, 'mike', '[e-mail]')
INSERT INTO #TableA(ID, NID, Email) VALUES (3, 'dave', '[e-mail]')

INSERT INTO #TableB(ID, NID, Email) VALUES (1, 'bob', '[e-mail]')
INSERT INTO #TableB(ID, NID, Email) VALUES (2, 'mike', '[e-mail]')
INSERT INTO #TableB(ID, NID, Email) VALUES (3, 'doug', '[e-mail]')

SELECT 
    #TableB.ID, 
    #TableB.NID, 
    #TableB.Email
FROM 
    #TableB Inner Join #TableA
    ON #TableA.ID = #TableB.ID WHERE #TableA.NID <> #TableB.NID

DROP TABLE #TableA
DROP TABLE #TableB

---

Result:

3 doug [e-mail]

---

In conclusion just a join with a WHERE condition

SELECT 
    TableB.ID, 
        TableB.NID, 
        TableB.Email
FROM 
        TableB 
Inner Join 
        TableA
ON TableA.ID = TableB.ID WHERE TableA.NID <> TableB.NID