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CASE / WHEN, IF, JOIN and COALESCE

Posted on 2009-05-04
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Last Modified: 2012-05-06
Hi,

I have three tables: Users, Customers and Staff

Users contains three columns: ID_Users, ID_Customers, and ID_Staff. ID_Users is the identity and primary key. In each row, either ID_Customers or ID_Staff will have a value, but never both.

Both Customers and Staff have columns named firstName and lastName as well as an identity field (ID_Staff / ID_Customers).

I want to be able to parse the Users table and get the following information back:
ID_Users, firstName, lastName and a text field identifying whether the row is a staff member or customer. Obviously, the source of firstName and lastName will differ depending on whether the user if a customer or staff member.

I just have no idea how to put it all together...
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Question by:Sanx69
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7 Comments
 
LVL 60

Expert Comment

by:chapmandew
ID: 24298566

something like this...

select u.id_users, u.id_customers, coalesce(c.firstname, s.firstname) as firstname, coalesce(c.lastname, u.lastname)....
from users u
left join customers c on u.id_users
left join staff s on u.id_staff = s.id_staff

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LVL 142

Accepted Solution

by:
Guy Hengel [angelIII / a3] earned 250 total points
ID: 24298570
this should do
SELECT u.ID_users, ISNULL(c.firstname, s.firstname) firstname, isnull(c.lastname, s.lastname) lastname
  FROM users u
  LEFT JOIN Customers c
    ON c.ID_Customer = u.ID_Customer
  LEFT JOIN Staff s
    ON s.ID_Staff = u.ID_Staff

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LVL 142

Expert Comment

by:Guy Hengel [angelIII / a3]
ID: 24298575
now, the obvious question is:
why don't you keep the "common" data (list firstname/lastname) in the central table (users)?
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LVL 60

Assisted Solution

by:chapmandew
chapmandew earned 250 total points
ID: 24298602
quick correction to mine...my fingers went to fast for me.

select u.id_users, u.id_customers, coalesce(c.firstname, s.firstname) as firstname, coalesce(c.lastname, u.lastname)....
from users u
left join customers c on u.id_customer = c.id_customer
left join staff s on u.id_staff = s.id_staff
0
 
LVL 3

Author Closing Comment

by:Sanx69
ID: 31577725
Thank you. The solutions differ only on the use of ISNULL versus COALESCE, and both solutions work just fine, so I've split the points evenly between you. Thanks very much for your help.
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LVL 3

Author Comment

by:Sanx69
ID: 24298959
@angelll

It's a good question. The Staff and Customers table contain a lot more besides just the names and most of the fields are not common. For the vast majority of operations, I'd be working on one table or the other; it's only select operations that I have to consider them jointly.
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LVL 3

Author Comment

by:Sanx69
ID: 24299092
Ooops. Didn't notice this was missing from both solutions. I was also looking for a method of determining whether the names returned were of a customer of staff member. And I managed to work it out all on my own, to my surprise:

SELECT    u.ID_users,
          ISNULL(c.firstname, s.firstname) firstname,
          ISNULL(c.lastname, s.lastname) lastname,
          CASE
              WHEN u.ID_Customer IS NULL
              THEN 'Staff' ELSE 'Customer'
              END customerType
FROM users u
LEFT JOIN Customers c
     ON c.ID_Customer = u.ID_Customer
LEFT JOIN Staff s
     ON s.ID_Staff = u.ID_Staff

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