Handling Null or Undef values in DBI (and things of those nature)...

data67 has asked for the wisdom of the Perl Monks concerning the following question:

Hi all,

Here is the deal guys. I am using DBI module with Oracle 8.1.7. (first timer) What i have is just an average web based interface for an oracle backend.
Two qestions:

1. How to handle my problem fields (empty fields?).
2. How to check if a record exists.

Question 1:
The problem is that when i try to pull in my data from the table my program crashes on the first field that is not defined.( i don't know if its a prblem with (NULL => undef ) or somthing else.

I have seen people use DBI::neat(undef) to handle similar thing and i tried that also with no luck, i still get "use of uninitialized value" message.
Here is what I have till now : (which by the way works good on records that have "all" fields filled.)


## connect to DB

## Run my SQL statements

while (@row = $sth->fetchrow_array) {

$employee_id                    = $row[0];
$last_name                      = $row[1];
$first_name                     = $row[2];
$cell_phone_number              = $row[3];
$email                          = $row[15];
$information_date               = $row[17];

### clean up for web## eq "");
$last_name                      = "&nbsp;" if ($last_name eq "");
$first_name                     = "&nbsp;" if ($first_name eq "");
$cell_phone_number              = "&nbsp;" if ($cell_phone_number eq "
+");
$email                          = "&nbsp;" if ($email eq "");
$information_date               = "&nbsp;" if ($information_date eq ""
+);

## then
print "$PRINT the "STUFF" retrieved";


## disconnect
[download]

As you can see at the moment, this is really plain. Question 2:
I also want to find out "how?" i can check a the table before i add new records to it.

##somthing like so

check if record exists {
    if not present{
        &add_new;
    } else {
        print " RECORD EXISTS";
    }
}
[download]

Comment on Handling Null or Undef values in DBI (and things of those nature)... Select or Download Code

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Re: Handling Null or Undef values in DBI (and things of those nature)... by mpeppler (Vicar) on Jan 08, 2002 at 05:26 UTC
For Q.2: In order to avoid race conditions (where you check fo r the record existence in the db, but someone else inserts it before you have a chance of inserting it) you should let the database do the checking. This is normally done with a constraint (usually a unique index of some sort). You then trap that error in the insert logic. Something like this: `# Assumes RaiseError is set eval { $dbh->do("insert ...."); }; if($@) { # check for the "duplicate insert" error here, and # handle accordingly... }` [download] Michael	[reply] [d/l]
Re: Handling Null or Undef values in DBI (and things of those nature)... by kwoff (Friar) on Jan 08, 2002 at 04:36 UTC
Question 1: grep `perldoc DBI` for "undef": undef NULL values are represented by undefined values in Perl You might check like: `$employee_id = (defined $row[0]) ? $row[0] : '';` [download] Question 2: do a select for that row. Using fetchrow_array(), it will return an empty list if there wasn't a row, so `@foo = $sth->fetchrow_array(); if (@foo) { ... } else { ... }` [download] I use MySQL, so maybe Oracle has fancier features, for which read the manual.	[reply] [d/l] [select]
Re: Handling Null or Undef values in DBI (and things of those nature)... by gav^ (Curate) on Jan 08, 2002 at 09:05 UTC
Or a slightly different syntax: `my ($field1, $field2, $field3) = (undef, undef, undef); my $sth = $dbh->prepare('select * from table'); $sth->execute; if ($sth->rows > 0) { $sth->bind_columns(\($field1, $field2, $field3)); $sth->fetch; } $sth->finish; for ($field1, $field2, $field3) { $_ \|\|= '' }` [download] When dealing with failures caused by duplicate values (due to a constraint) I just check like this: `$dbh->do('insert into blah (?, ?, ?)', undef, $x, $y, $z); if ($dbh->err) { if ($dbh->err == 1062) { # duplicate } else { # some other problem } }` [download] 1062 happens to be the error code for MySQL. Look at $DBI::lasth->{Statement} which has the last SQL statement you tried to execute which is quite handy in a debug subroutine.	[reply] [d/l] [select]
Re: Handling Null or Undef values in DBI (and things of those nature)... by rbc (Curate) on Jan 08, 2002 at 04:49 UTC
I hope this info can help you with your 2nd question. Off the top of my head there are aleast 3 ways to check the table before you insert a new record. way one) would be to define a unique constraint on this table in the DBMS. This will prevent any insertion of a record that is a duplicate. way two) would be to define a before insert trigger and have this trigger check for a duplicate. way three) execute the query *select count() into n from ...** and if n > 0 then the record exist. -- Its like a dog that can sing and dance. It's remarkable because it can do it. Not that it can do it well.	[reply]
Re: Re: Handling Null or Undef values in DBI (and things of those nature)... by data67 (Monk) on Jan 08, 2002 at 05:10 UTC
Can you please give me an example or two on your seggestions. As far as my first question i got it done by doing this `while (@row = $sth->fetchrow_array) { foreach (@$row) { $_ = '' unless defined; ### HANDLE NULL FIELDS } }` [download]	[reply] [d/l]
Re: Re: Handling Null or Undef values in DBI (and things of those nature)... by rbc (Curate) on Jan 09, 2002 at 02:28 UTC
Find an SQL manual or on-line doc. and look up the ALTER TABLE and CREATE TRIGGER commands. They do a much better job than I could :) -- Its like a dog that can sing and dance. It's remarkable because it can do it. Not that it can do it well.	[reply]


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