Question

I have an application where a user adds things they’ve bought to their account. They create a list name e.g. my ebay purchaes, then add what they’ve bought in the past.

When they create the list they can choose whether its public or not, and this is where i’m struggling.

I need to show a list of products and who listed them onto which list, but, if they were listed onto a private list then show the users name as private and the list name as private

4 tables:

products

prod_id    store_name   product    user_id
-------    ---------    -------    -------
1          ebay         chair       1002
2          amazon       desk        1000
3          ebay         lamp        1001

users

user_id    name
-------    -------
1000       john
1001       mark
1002       sue

user_onto_list

user_onto_list_id    user_id      list_id
-------              ---------    -------
1                    1000         11
2                    1001         12
3                    1002         10

list_detail

list_id    visibility    list_name
-------    -------       ---------
10         open          myEbayList
11         open          myAmazonList
12         private       myPrivateList

so the result would be:

product    listed_by    list_name
-------    ---------    ---------
chair      sue          myEbayList
desk       john         myAmazonList
lamp       PRIVATE      PRIVATE

Is this possible in one query or would you redesign how the data was stored to make it simpler? I’d rather keep it like this so if e.g sue decided to make her myEbatList private at any time the products would not show as being listed by her and mark may decide to make his list public.

Advice appreciated

Answer 1

select
p.product
, case when ld.visibility = 'private' then
   'PRIVATE'
  else
   u.name
  end as listed_by
, case when ld.visibility = 'private' then
   'PRIVATE'
  else
   ld.list_name
  end as listed_name
from products p

inner join users u on u.ser_id = p.user_id
inner join user_onto_list uol on uol.user_id = u.user_id
inner join list_detail ld on ld.list_id = uol.list_id

EDIT: I’ve included the logic regarding privacy in CASE statements as part of the SELECT, as this does not interfere with the JOIN criteria, it’s only display information. Alternatively you could build that logic into some kind of UNION structure in a derived table and then join to that, but that would probably be less inefficient.