Thread: Getting pk of the most recent row, in a group by
I've got a table of "coupons" which have an expiration date. For each type of coupon, I'd like to get the primary key of the coupon which will expire first. # create table coupon ( coupon_id serial primary key, type varchar(255), expires date ); insert into coupon values(DEFAULT,'free','2007-01-01'); insert into coupon values(DEFAULT,'free','2007-01-01'); insert into coupon values(DEFAULT,'free','2007-06-01'); insert into coupon values(DEFAULT,'free','2007-06-01'); insert into coupon values(DEFAULT,'50%','2008-06-01'); insert into coupon values(DEFAULT,'50%','2008-06-02'); insert into coupon values(DEFAULT,'50%','2008-06-03'); The desired query would look like: # select coupon_id,type,expires from coupon where type='free' order by expires limit 1;coupon_id | type | expires -----------+------+------------ 1 | free | 2007-01-01 But be grouped by type: # select type,min(expires),count(*) from coupon group by type;type | min | count ------+------------+-------free | 2007-01-01 | 4 ; pk=150% | 2008-06-01 | 3 ; pk=5 In the second example, is it possible to get the primary key of the row with the minimum expires time? -- ---- Visit http://www.obviously.com/
On 8/13/07, Bryce Nesbitt <bryce1@obviously.com> wrote: > In the second example, is it possible to get the primary key of the row > with the minimum expires time? SELECT TYPE, MIN(expires), COUNT(*) , (SELECT MIN(coupon_id) FROM coupon WHERE expires = MIN(c.expires))AS coupon_id FROM coupon c GROUP BY TYPE;
On Aug 13, 2007, at 15:05 , Bryce Nesbitt wrote: > # select type,min(expires),count(*) from coupon group by type; > type | min | count > ------+------------+------- > free | 2007-01-01 | 4 ; pk=1 > 50% | 2008-06-01 | 3 ; pk=5 > > In the second example, is it possible to get the primary key of the > row > with the minimum expires time? I believe DISTINCT ON will do what you want, if you don't mind using non-SQL-spec functionality: SELECT DISTINCT ON (type)type, expires, coupon_id FROM coupon ORDER BY type, expires; I believe you'd need to add the COUNT using a join: SELECT type, expires, coupon_id, type_count FROM ( SELECT DISTINCT ON (type) type, expires, coupon_id FROM coupon ORDER BY type, expires )earliest_to_expire JOIN ( SELECT type, count(coupon_id) as type_count FROM coupons GROUP BY type ) type_counts USING (type); Michael Glaesemann grzm seespotcode net
Do you have a table of coupon types? Terry Terry Fielder terry@greatgulfhomes.com Associate Director Software Development and Deployment Great Gulf Homes / Ashton Woods Homes Fax: (416) 441-9085 Bryce Nesbitt wrote: > I've got a table of "coupons" which have an expiration date. For each > type of coupon, I'd like to get the primary key of the coupon which will > expire first. > > # create table coupon > ( > coupon_id serial primary key, > type varchar(255), > expires date > ); > insert into coupon values(DEFAULT,'free','2007-01-01'); > insert into coupon values(DEFAULT,'free','2007-01-01'); > insert into coupon values(DEFAULT,'free','2007-06-01'); > insert into coupon values(DEFAULT,'free','2007-06-01'); > insert into coupon values(DEFAULT,'50%','2008-06-01'); > insert into coupon values(DEFAULT,'50%','2008-06-02'); > insert into coupon values(DEFAULT,'50%','2008-06-03'); > > The desired query would look like: > > # select coupon_id,type,expires from coupon where type='free' order by > expires limit 1; > coupon_id | type | expires > -----------+------+------------ > 1 | free | 2007-01-01 > > > But be grouped by type: > > # select type,min(expires),count(*) from coupon group by type; > type | min | count > ------+------------+------- > free | 2007-01-01 | 4 ; pk=1 > 50% | 2008-06-01 | 3 ; pk=5 > > In the second example, is it possible to get the primary key of the row > with the minimum expires time? > >