How can I write an update trigger to update the current table based on a value in another table in SQL Server 2008?

Question

I have two tables, Table_A and Table_B. Table_A has a one-to-many relationship with Table_B.

After inserting a record in the child table (Table_B) I want to use a trigger to grab a value from a field in the parent table (Table_A) and update the currently inserted record in Table_B.

Table_A

PK|EmpID|MemID|Firstname|Lastname
----------------------------------
1 |1234 |AB123|John     | Doe
2 |5678 |CD456|Jane     | Smith

Table_B

PK|EmpID|MemID|Description
---------------------
1 |1234 |NULL |Finance
2 |1234 |NULL |IT
3 |5678 |NULL |Finance 
4 |5678 |NULL |Management

The trigger should grab the MemID value from Table_A and update it's corresponding value in Table_B where [Table_A].[EmpID] = [Table_B].[EmpID]. Of course after 4 inserts in Table_B the results should be as follows:

PK|EmpID|MemID|Description
---------------------
1 |1234 |AB123|Finance
2 |1234 |AB123|IT
3 |5678 |CD456|Finance 
4 |5678 |CD456|Management

I extensively searched other technical sites but can't seem to find one with this particular scenario. As a desperate move I have registered into this site and this would be my 1st post for help.

The following is my futile attempt at trying to create my 1st trigger in MS SQL Server 2008 R2.

CREATE TRIGGER dbo.trgInsMemID
ON dbo.Table_B
AFTER INSERT
AS
    BEGIN
        UPDATE dbo.Table_B
            SET MemID = 
                (SELECT MemID 
                FROM inserted i 
                JOIN dbo.Table_A c ON c.EmpID = i.EmpID)
            FROM dbo.Table_B b
            JOIN inserted i ON i.MemID = b.MemID
    END

Answer 1

I believe you'll want your update statement changed to look like this:

UPDATE b
SET b.MemId = a.MemId
FROM dbo.Table_B AS b
INNER JOIN Inserted AS i ON b.EmpId = i.EmpId
INNER JOIN dbo.Table_A AS a ON b.EmpId = a.EmpId

This will update only the records in b that were also in the insterted table. (first INNER JOIN)

It will then INNER JOIN into Table_A on EmpId to pull the proper MemId to update your records with.