Обсуждение: [Q] Table aliasing
Hello.
I have a noob question about table aliases in SQL.
Suppose the following query:
SELECT *
FROM deal
WHERE
deal.id IN (
SELECT DISTINCT deal.id
FROM
deal
JOIN partner_share ON deal.id = partner_share.deal_id
JOIN partner ONshare.partner_id = partner.id
WHERE
partner.team_id = 12345
AND (
partner_share.type = 1 AND deal.external_id IS NOT NULL
OR partner_share.type = 2 AND deal.external_id IS NULL
)
)
As you can see, the IN (...) references the deal table too.
My questions:
* 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 (...).
Thank you,
L.
I almost always alias my tables by default with something short (Usually 1 - 3 characters), but not my subselects for an in list. In this case I would do d1, d2, ps, and p for the different tables. I then do my best to use the same alias in all my queries. I am also big on formatting the SQL here is how I would write what you have for readability
INNER
JOIN partner_share AS ps
JOIN partner_share AS ps
INNER
JOIN partner AS p
JOIN partner AS p
ON ps.partner_id = p.partner.id
WHERE p.team_id = 12345
AND (ps.type = 1 AND d2.external_id IS NOT NULL
OR ps.type = 2 AND d2.external_id IS NULL)
)
WHERE p.team_id = 12345
AND (ps.type = 1 AND d2.external_id IS NOT NULL
OR ps.type = 2 AND d2.external_id IS NULL)
)
On Fri, Oct 4, 2013 at 5:59 AM, Ladislav Lenart <lenartlad@volny.cz> wrote:
Hello.
I have a noob question about table aliases in SQL.
Suppose the following query:
SELECT *
FROM deal
WHERE
deal.id IN (
SELECT DISTINCT deal.id
FROM
deal
JOIN partner_share ON deal.id = partner_share.deal_id
JOIN partner ONshare.partner_id = partner.id
WHERE
partner.team_id = 12345
AND (
partner_share.type = 1 AND deal.external_id IS NOT NULL
OR partner_share.type = 2 AND deal.external_id IS NULL
)
)
As you can see, the IN (...) references the deal table too.
My questions:
* 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 (...).
Thank you,
L.
--
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general
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. David J. -- View this message in context: http://postgresql.1045698.n5.nabble.com/Q-Table-aliasing-tp5773355p5773364.html Sent from the PostgreSQL - general mailing list archive at Nabble.com.
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