New to SQLite, very rusty on DBI and SQL. I'm trying to build a table of unique values, and I need to know their rowids so that other tables can refer to these values, and bring them together with a SELECT .. JOIN. Every time I store a value in the table, I need to first check to see if the value already exists -- if so, I need to obtain its rowid; if not, I need to insert it and then obtain its rowid.

This test program shows how I've tried to do it: 

#!/usr/bin/env perl

use v5.38;
use DBI;
my $memory;    # = ':memory:';
my $dbname = $memory // '/Users/chap/private/data/music';
my $dbh    = create_database();

# insert some songs. watch out for dupes.
for my $song (qw{ Alpha Beta Gamma Alpha Delta Omicron }) {
    my $rowid = insert_unique( 'Songs', 'song', $song );
    say "Inserting '$song', got rowid=$rowid";
}

sub create_database() {
    unlink $dbname unless $memory;
    my $dbms   = 'SQLite';
    my $dbname = $dbname;
    my $user   = '';
    my $passwd = '';
    my $dbh =
        DBI->connect( "DBI:$dbms:dbname=$dbname", $user, $passwd,
        { RaiseError => 1, AutoCommit => 1 } )
        or die "Cannot connect: $DBI::errstr";

    my $songs = <<~'EOF';
    CREATE TABLE Songs (id        INTEGER PRIMARY KEY,
                 song             VARCHAR(40) UNIQUE
                 )
    EOF
    $dbh->do($songs);
    $dbh->do("CREATE INDEX ix_song ON Songs (song)");

    return $dbh;
}

# sub which conditionally inserts a unique value, and returns rowid
sub insert_unique( $table, $column, $value ) {
    state %sths; # cache my prepared statement handles
    if ( !exists $sths{$table} ) {
        $sths{$table}{SELECT} =
            $dbh->prepare("SELECT id FROM $table WHERE $column = (?)")
+;
        $sths{$table}{INSERT} =
            $dbh->prepare("INSERT INTO $table ($column) VALUES (?)");
    }
    my $rowid = $sths{$table}{SELECT}->execute($value) || die;
    say "Value='$value': attempt to select yielded rowid=$rowid";
    if ( $rowid == 0 ) {
        say "So value needs to be inserted.";
        $sths{$table}{INSERT}->execute($value);
        $rowid = $dbh->do("SELECT last_insert_rowid()");
    }
    else {
        say "So value has been previously inserted.";
    }
    return $rowid;
}

[download]
Running produces this output: 
Value='Alpha': attempt to select yielded rowid=0E0
So value needs to be inserted.
Inserting 'Alpha', got rowid=1
Value='Beta': attempt to select yielded rowid=0E0
So value needs to be inserted.
Inserting 'Beta', got rowid=1
Value='Gamma': attempt to select yielded rowid=0E0
So value needs to be inserted.
Inserting 'Gamma', got rowid=1
Value='Alpha': attempt to select yielded rowid=0E0
So value needs to be inserted.
DBD::SQLite::st execute failed: UNIQUE constraint failed: Songs.song a
+t /Users/chap/private/perl/insert_unique line 51.
DBD::SQLite::st execute failed: UNIQUE constraint failed: Songs.song a
+t /Users/chap/private/perl/insert_unique line 51.

[download]
Several things were not as expected: executing 'SELECT id...' returned 0e0 even when the value exists in the table (as in adding Alpha the 2nd time). And when the value needed to be inserted, the call to `last_insert_rowid()` always returned 1, rather than the most recently inserted rowid.

The following shows that the data was inserted into the table: 

$ sqlite3 music
SQLite version 3.43.2 2023-10-10 13:08:14
Enter ".help" for usage hints.
sqlite> select * from Songs;
1|Alpha
2|Beta
3|Gamma

[download]
In addition to being baffled why this isn't working as I expected, I also wonder if there isn't a simpler way to accomplish this that I haven't thought of. I would imagine that this is a common situation when loading a normalized database.

Humbly yours,

1620

In reply to SQLite: INSERT into a unique column and retrieve rowid by ibm1620

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