it's explained pretty well on the book

http://web2py.com/books/default/chapter/29/06/the-database-abstraction-layer#belongs

let me know if something is not clear

Il giorno mercoledì 25 settembre 2013 07:11:27 UTC+2, step ha scritto:
>
> Niphlod, can you please explain why you're using method _select instead of 
> select in your code below? In there no need to connect to SQL to set 
> all_courses?
>
> On Tuesday, September 24, 2013 3:35:34 PM UTC+2, Niphlod wrote:
>>
>> you can do that with
>>
>> all_courses = db(db.courses.id>0)._select(db.courses.memberid)
>> all_members_not_in_courses = db(~db.members.id.belongs(all_courses))
>>
>> you can eventually throw in a distinct on db.courses.memberid if the 
>> number of courses rows is plenty.
>>
>> Il giorno martedì 24 settembre 2013 12:16:52 UTC+2, Michael Hall ha 
>> scritto:
>>>
>>> I am not the most awesome SQL person and as such I am having trouble 
>>> constructing a query I need.
>>>
>>> I have two tables in a database called "old". The parent table is called 
>>> "members" and there is a child table called "courses".
>>>
>>> #--------
>>> old.define_table('members',
>>>     Field('id'),
>>>     Field('profile_id','string'),
>>>     Field('title','string'),
>>>     Field('firstname','string'),
>>>     Field('surname','string'),
>>>     Field('memberlevel','string'),
>>>     Field('lawfirm','string'),
>>>     Field('registerdate','date'),
>>>     Field('expirydate','date'),
>>>     Field('email','string'),
>>>     Field('telno','string'),
>>>     Field('address1','string'),
>>>     Field('address2','string'),
>>>     Field('address3','string'),
>>>     Field('town','string'),
>>>     Field('county','string'),
>>>     Field('postcode','string'),
>>>     Field('country','string'),
>>>     Field('notes','text'),
>>>     Field('memberstatus','string'),
>>>     migrate=False)
>>>
>>> old.define_table('courses',
>>>     Field('memberid',type='reference members',),
>>>     Field('profile_id','string'),
>>>     Field('course','string'),
>>>     Field('enroldate','string'),
>>>     migrate=False)
>>>
>>> I would like to select all records in the members table that are not 
>>> referenced by records in the courses table. I suppose it could be described 
>>> as the opposite of a join as I only want to select members that do not have 
>>> courses attached to them.
>>>
>>> Any help would be appreciated.
>>>
>>>
>>>

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