Firebird select from table distinct one field - sql

Question

The question I asked yesterday was simplified but I realize that I have to report the whole story.
I have to extract the data of 4 from 4 different tables into a Firebird 2.5 database and the following query works:
SELECT
PRODUZIONE_T t.CODPRODUZIONE,
PRODUZIONE_T.NUMEROCOMMESSA as numeroco,
ANGCLIENTIFORNITORI.RAGIONESOCIALE1,
PRODUZIONE_T.DATACONSEGNA,
PRODUZIONE_T.REVISIONE,
ANGUTENTI.NOMINATIVO,
ORDINI.T_DATA,
FROM PRODUZIONE_T
LEFT OUTER JOIN ORDINI_T ON PRODUZIONE_T.CODORDINE=ORDINI_T.CODORDINE
INNER JOIN ANGCLIENTIFORNITORI ON ANGCLIENTIFORNITORI.CODCLIFOR=ORDINI_T.CODCLIFOR
LEFT OUTER JOIN ANGUTENTI ON ANGUTENTI.IDUTENTE = PRODUZIONE_T.RESPONSABILEUC
ORDER BY right(numeroco,2) DESC, left(numeroco,3) desc
rows 1 to 500;
However the query returns me double (or more) due to the REVISIONE column.
How do I select only the rows of a single NUMEROCOMMESSA with the maximum REVISIONE value?

Answer 1

This should work:
select COD, ORDER, S.DATE, REVISION
FROM TAB1
JOIN
(
select ORDER, MAX(REVISION) as REVISION
FROM TAB1
Group By ORDER
) m on m.ORDER = TAB1.ORDER and m.REVISION = TAB1.REVISION

Answer 2

Here you go - http://sqlfiddle.com/#!6/ce7cf/4
Sample Data (as u set it in your original question):
create table TAB1 (
cod integer primary key,
n_order varchar(10) not null,
s_date date not null,
revision integer not null );
alter table tab1 add constraint UQ1 unique (n_order,revision);
insert into TAB1 values ( 1, '001/18', '2018-02-01', 0 );
insert into TAB1 values ( 2, '002/18', '2018-01-31', 0 );
insert into TAB1 values ( 3, '002/18', '2018-01-30', 1 );
The query:
select *
from tab1 d
join ( select n_ORDER, MAX(REVISION) as REVISION
FROM TAB1
Group By n_ORDER ) m
on m.n_ORDER = d.n_ORDER and m.REVISION = d.REVISION
Suggestions:
Google and read the classic book: "Understanding SQL" by Martin Gruber
Read Firebird SQL reference: https://www.firebirdsql.org/file/documentation/reference_manuals/fblangref25-en/html/fblangref25.html
Here is yet one more solution using Windowed Functions introduced in Firebird 3 - http://sqlfiddle.com/#!6/ce7cf/13
I do not have Firebird 3 at hand, so can not actually check if there would not be some sudden incompatibility, do it at home :-D
SELECT * FROM
(
SELECT
TAB1.*,
ROW_NUMBER() OVER (
PARTITION BY n_order
ORDER BY revision DESC
) AS rank
FROM TAB1
) d
WHERE rank = 1
Read documentation
https://community.modeanalytics.com/sql/tutorial/sql-window-functions/
https://www.firebirdsql.org/file/documentation/release_notes/html/en/3_0/rnfb30-dml-windowfuncs.html
Which of the three (including Gordon's one) solution would be faster depends upon specific database - the real data, the existing indexes, the selectivity of indexes.
While window functions can make the join-less query, I am not sure it would be faster on real data, as it maybe can just ignore indexes on order+revision cortege and do the full-scan instead, before rank=1 condition applied. While the first solution would most probably use indexes to get maximums without actually reading every row in the table.
The Firebird-support mailing list suggested a way to break out of the loop, to only use a single query: The trick is using both windows functions and CTE (common table expression): http://sqlfiddle.com/#!18/ce7cf/2
WITH TMP AS (
SELECT
*,
MAX(revision) OVER (
PARTITION BY n_order
) as max_REV
FROM TAB1
)
SELECT * FROM TMP
WHERE revision = max_REV

Answer 3

If you want the max revision number in Firebird:
select t.*
from tab1 t
where t.revision = (select max(t2.revision) from tab1 t2 where t2.order = t.order);
For performance, you want an index on tab1(order, revision). With such an index, performance should be competitive with any other approach.