Re: SQL Query Newbie Help

On Fri, 24 Mar 2006, Julie Robinson wrote:

> This works, but is there a better solution?
>
> select *
> from quality_control_reset T
> where date = (
> select max(date)
> from quality_control_reset
> where qualitycontrolrange = T.qualitycontrolrange);

If you can use PostgreSQL extensions (and don't care that you might not
get two rows if two ids had the same date equaling the max date for a
given range), maybe something like:

select distinct on (qualitycontrolrange) id, date, qualitycontrolrange
from quality_control_reset order by qualitycontrolrange,date desc;

Otherwise, you might see how the above compares in plan to something like
(not really tested):

select T.* from quality_control_reset T inner join
(select qualitycontrolrange, max(date) as date from quality_control_reset
group by qualitycontrolrange) T2
on (T.qualitycontrolrange = T2.qualitycontrolrange and T.date=T2.date);

> Julie Robinson wrote:
> > Given the two tables at the bottom of this email, I'm having trouble
> > coming up with a SQL statement that returns all rows in the
> > quality_control_reset table where there is only one row for the most
> > recent quality_control_range. Help?
> >
> > Example:
> >
> > In table quality_control_reset:
> >
> > id | date | qualitycontrolrange
> > ---------------------------------------------
> > 1 | 02/23/2006 | 20
> > 2 | 02/23/2006 | 6
> > 3 | 02/28/2006 | 18
> > 4 | 03/01/2006 | 18
> > 5 | 03/23/2006 | 12
> > 6 | 03/23/2006 | 20
> >
> > I want the results of the following from the query:
> >
> > id | date | qualitycontrolrange
> > ---------------------------------------------
> > 2 | 02/23/2006 | 6
> > 4 | 03/01/2006 | 18
> > 5 | 03/23/2006 | 12
> > 6 | 03/23/2006 | 20
> >
> >
> > CREATE TABLE quality_control_reset
> > (
> > id int8 NOT NULL,
> > date timestamp,
> > qualitycontrolrange int8,
> > CONSTRAINT quality_control_reset_pkey PRIMARY KEY (id),
> > CONSTRAINT fk42a706efb62efa94 FOREIGN KEY (qualitycontrolrange)
> > REFERENCES quality_control_range (id) MATCH SIMPLE
> > ON UPDATE NO ACTION ON DELETE NO ACTION
> > )
> >
> > CREATE TABLE quality_control_range (
> > id int8 NOT NULL,
> > code varchar(255),
> > CONSTRAINT quality_control_range_pkey PRIMARY KEY (id)
> > );