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Re: DBI: select distinct returns same values

by FreeBeerReekingMonk (Deacon)
on Sep 21, 2019 at 11:08 UTC ( #11106474=note: print w/replies, xml ) Need Help??


in reply to DBI: select distinct returns same values

The "date" is probably a timedate, and eventhough you see it as the only-the-date, the time makes it unique. Try to put also a groupby clause:
select distinct cast(DATE as date) from myschema.mytable GROUP BY DATE

untested.

Edit: somewhat tested on https://www.jdoodle.com/execute-sql-online/

create table foo (IDENTITY INT, LoginTime DATETIME); INSERT INTO foo VALUES( 1, '2019-09-29 17:00:48.000' ); INSERT INTO foo VALUES( 2, '2019-09-29 17:29:02.000' ); INSERT INTO foo VALUES( 3, '2019-09-30 10:22:46.000' ); INSERT INTO foo VALUES( 4, '2019-09-30 11:16:23.000' ); --Select unique SELECT distinct cast(LoginTime as date) FROM foo GROUP BY LoginTime;

returns: 2019 (just the year, strange)

EDIT 2: Ok, tried sqlite from Perl and SQLite does not have DATE... and returns the same "2019" as the online version. But it DOES have strftime and that made it work

#!/usr/bin/perl use strict; use warnings; use DBI; my $dbfile = "sample.db"; my $dsn = "dbi:SQLite:dbname=$dbfile"; my $user = "x"; my $password = "x"; my $dbh = DBI->connect($dsn, $user, $password, { PrintError => 0, RaiseError => 1, AutoCommit => 1, FetchHashKeyName => 'NAME_lc', }); if(-z $dbfile){ my $sql = <<'END_SQL'; CREATE TABLE foo ( id INTEGER PRIMARY KEY, LoginTime DATE ) END_SQL $dbh->do($sql); my @DATES = ('2019-09-29 17:00:48.000', '2019-09-29 17:29:02.000', ' +2019-09-30 10:22:46.000', '2019-09-30 11:16:23.000'); for(my $i=0;$i<=$#DATES;$i++){ $dbh->do('INSERT INTO foo (LoginTime) VALUES (?)', undef,$DATES[$i]); } } # This is what I thought would work #my $sql = 'select distinct cast(LoginTime as date) from foo GROUP BY +LoginTime'; # this actually works in SQLite (strftime) my $sql = 'select distinct strftime("%d-%m-%Y",LoginTime) from foo'; my $sth = $dbh->prepare($sql); $sth->execute(); while (my @row = $sth->fetchrow_array) { print "[@row]\n"; } $dbh->disconnect;

outputs:

[29-09-2019] [30-09-2019]

Ok, after reading @Erix's comment, he's right. And Anonymous Monk (too bad for the points) pointed the error out, and it also occurs with SQLite. Data::Dumper shows the following after injecting 29th then 30th:

$VAR1 = [ [ '30-09-2019' ], $VAR1->[0] ];

So all old values become a reference to the first entry of itself. So if you inject n fetchrow_arrayref() then you get n times the last pushed entry.
So to copy an arrayref: [@$ref] to get an AoA in @all_rows.

$sth->execute(); my @all_rows; while ( my $ref = $sth->fetchrow_arrayref()) { # Needs to be de-referenced (Anonymous monk's fix) push(@all_rows, [@{$ref}]); } foreach my $row_ref (@all_rows) { print "$row_ref->[0]\n"; }

And my first example worked because I used fetchrow_array() instead of fetchrow_arrayref() and went around the referencing problem (but references are faster in general, not by much, so keep using fetchrow_arrayref)

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Re^2: DBI: select distinct returns same values
by erix (Parson) on Sep 21, 2019 at 14:02 UTC

    select distinct cast(DATE as date) from myschema.mytable GROUP BY DATE

    Sorry, but that GROUP BY really doesn't make sense. No point in using the GROUP BY clause here.

Re^2: DBI: select distinct returns same values
by tukusejssirs (Sexton) on Sep 21, 2019 at 13:04 UTC

    Thank you for your answer!

    You presumed right that date contains datetime. However, I have no idea why do you think I use sqliteI said I have a Pg (i.e. PostgreSQL) database table. :)

    The thing is that Pg does not support strftime() function.

    So, according to your answer I believe that DBI does not get the selected values (i.e. the output from select distinct cast(...)) or it does not understand/use the cast() function, instead it selects the timestamp value (the actual value) instead of the casted one. Is that right? Is this expected? I believe it is not. And if it is so, I would say it is a bug. What do you think?

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