On 4.10.2013 15:11, David Johnston wrote:
> Ladislav Lenart wrote
>> * Should I alias one of the references to deal?
>> * The above query works the same both with and without an alias, so is it
>> simply
>> a matter of taste / good practice?
>> * Where can I find more info about this, i.e. when the alias is mandatory
>> and
>> when it is only a convenience? I've scanned through
>>
>>
>> http://www.postgresql.org/docs/9.1/static/queries-table-expressions.html
>>
>> but it only mentions that subselect in FROM must have an alias. I would
>> like to
>> know about IN (...) and EXISTS (...).
>
> Aliases are mandatory for self-joins. Basically two relations at the same
> query level cannot have the same name. With sub-selects the inner level
> shields the outer level from being visible so an alias is optional. I
> believe the case of a correlated sub-query requires the alias for the same
> reason - otherwise you end up with a always true where clause when joining
> the supposed two deal tables together.
>
> There is no right/best way but it is always more clear to rename such that
> you avoid repeating the same relation name in the query.
I think I understand.
Thank you,
Ladislav Lenart