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Re^2: return primary key if duplicate entry exists?

by diyaz (Beadle)
on Jan 24, 2016 at 15:44 UTC ( [id://1153507]=note: print w/replies, xml ) Need Help??


in reply to Re: return primary key if duplicate entry exists?
in thread return primary key if duplicate entry exists?

I see. I just assumed if SQL is already looking for a duplicate then maybe it can also just give me the id/key of the row. Seems inefficient to have that as separate steps. Since they have last_insert_id, I assumed they would have a function similar.

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Re^3: return primary key if duplicate entry exists?
by diyaz (Beadle) on Jan 24, 2016 at 15:50 UTC
    actually couldn't I allow it to replace the duplicate with the duplicate and that way last_insert_id will return that row id for me?

      Here's a working example. It does return the id either of the new record or the existing record. Unfortunately it also appears to change the auto-increment value with every update (even when no new records are inserted) so id's may not be continuous.

      From the mysql docs :  In general, you should try to avoid using an ON DUPLICATE KEY UPDATE clause on tables with multiple unique indexes.

      #!\C:\Strawberry\perl\bin\perl use strict; use warnings; use DBI; # CREATE TABLE test2 ( # id int(11) NOT NULL AUTO_INCREMENT, # f1 varchar(45) DEFAULT NULL, # PRIMARY KEY (id), # UNIQUE KEY f1_UNIQUE (f1) # ) ENGINE=InnoDB AUTO_INCREMENT=27 DEFAULT CHARSET=utf8; my $table = 'test2'; my $col = 'f1'; # unique key my $entry = $ARGV[0] || 1; #PERL DBI CONNECT my $dbh = dbh(); my $sql = sprintf ' INSERT INTO %s (%s) VALUES (?) ON DUPLICATE KEY UPDATE id = LAST_INSERT_ID(id)', $dbh->quote_identifier($table), $dbh->quote_identifier($col) ; my $sth = $dbh->prepare($sql); my $rv = $sth->execute($entry); print $rv.' '.$dbh->last_insert_id(undef,undef,$table,$col); sub dbh{ my $database = "test"; my $user = "user"; my $pw = "password"; my $dsn = "dbi:mysql:$database:localhost:3306"; my $dbh = DBI->connect($dsn, $user, $pw, { RaiseError=>1, AutoCommit=>1 } ); return $dbh; }
      poj
        What is the use of 'UPDATE id = LAST_INSERT_ID(id)'?

        According to the MySQL docs LAST_INSERT_ID(id) will return the value of the argument, so you are actually replacing the existing value in the field 'id' by itself and then you are using that value as the next auto-incremented key value. That is likely to create even more errors as now the database doesn't know anymore where it was in the sequence it was using and will happily re-use an already existing number.

        if expr is given as an argument to LAST_INSERT_ID(), the value of the argument is returned by the function and is remembered as the next value to be returned by LAST_INSERT_ID().

        CountZero

        A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James

        My blog: Imperial Deltronics
        thanks! actually I didn't realize you can use placeholders with quote identifiers. For some reason I thought they had to be used separately.

        So I didn't fully comprehend my own code as some of it was borrowed. If I understand the code correctly, id will be equal to the new auto-increment id, which also means I can call that id from last_insert_id()? Also if it wasn't a duplicate, the last_insert_id() would also have the new key? So basically, with this I can always rely on last_insert_id() to give me the key?
      Actually, if you perform an INSERT which results in a duplicate key error, then by definition you must know the key already. How else could you have inserted that record otherwise?

      The last_insert_id is only useful with auto-incrementing keys (and even then I find it rarely needed), but in any case auto-incrementing keys should never give a duplicate key error.

      CountZero

      A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James

      My blog: Imperial Deltronics
        that is a good point, and probably a good reason to use unique values as primary keys. I guess I am slightly confused on database design then. I am working with DNA sequences and they can be quite long strings. So I am using a crc32 to check for uniqueness. Would it still be a good idea to use that as a primary key? I merely assumed that was not an ideal key?

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