I am new to Perl, and I am writing a script to fetch some rows from a database:
my #rows = $conn->fetchrow_array(1,2,3);
the result will be three rows of single column.
12345
56789
12376
How should I join them together as 12345,56789,56789
I tried,
my $list = join ",", #rows.
Result: ARRAY(0x14f6de0),ARRAY(0x1508a90),ARRAY(0x15014c0)
Going through a foreach loop just print the results with a new line:
12345
56789
12376
What am I doing wrong ? have I got the concept of fetchrow_array wrong?
Each row is a reference to an array (because each row could contain multiple columns). Something like the following should work.
#!/usr/bin/perl
use strict; use warnings;
my #rows = (
[ 12345 ],
[ 56789 ],
[ 12376 ],
);
my #vals = map #$_, #rows;
print join(',', #vals), "\n";
However, you are better off using selectcol_arrayref:
This utility method combines "prepare", "execute", and fetching one column from all the rows, into a single call. It returns a reference to an array containing the values of the first column from each row.
Related
I have hash with multiple values to a key. How to print multiple values of key in a hash independently?
# HASH with multiple values for each key
my %hash = ( "fruits" => [ "apple" , "mango" ], "vegs" => ["Potato" , "Onion"]);
# SET UP THE TABLE
print "<table border='1'>";
print "<th>Category</th><th>value1</th><th>value2</th>";
#Print key and values in hash in tabular format
foreach $key (sort keys %hash) {
print "<tr><td>".$key."</td>";
print "<td>".#{$hash{$key}}."</td>";
}
* Current Output: *
Category Value1 Value2
fruits apple mango
vegs Potato Onion
* Desired Output: *
Category Value1 Value2
fruits apple mango
vegs Potato Onion
Try replacing the second line of your loop with
print "<td>$_</td>" for #{ $hash{$key} };
Which will loop over each item in the array reference and wrap them in td tags.
I know I can do this with interpolation. Can I do it using placeholders?
I am getting this error:
DBD::Pg::st execute failed: ERROR: invalid input syntax for integer: "{"22,23"}" at ./testPlaceHolders-SO.pl line 20.
For this script:
#!/usr/bin/perl -w
use strict;
use DBI;
# Connect to database.
my $dbh = DBI->connect("dbi:Pg:dbname=somedb;host=localhost;port=5432", "somedb", "somedb");
my $typeStr = "22,23";
my #sqlParms = [ $typeStr ];
my $sqlStr = << "__SQL_END";
SELECT id
FROM states
WHERE typeId in (?)
ORDER BY id;
__SQL_END
my $query = $dbh->prepare($sqlStr);
$query->execute(#sqlParms);
my $id;
$query->bind_columns(\$id);
# Process rows
while ($query->fetch())
{
print "Id: $id\n";
}
Is there a way around it besides interpolation?
DBD::PG has support for PostgreSQL arrays, so you can simply write a query like this:
WHERE typeid = ANY( ARRAY[1,2,3] )
or, with a parameter...
WHERE typeid = ANY(?)
Then just use the array support
my #targets = (1,2,3);
# ...
$query->execute(\#targets);
Posting comment as answer, as requested.
Generate your own placeholder string. Like so:
my #nums = (22,23);
my $placeholder = join ",", ("?") x #nums;
$query->execute(#nums);
Yes. You must use placeholders for each value, such as IN (?, ?, ?). You can however generate the correct number of question marks using something like this (untested):
my #values = (22, 23, ...);
# will produce "?, ?, ..."
my $in = join ", ", ("?") x #values;
my $sqlStr = "SELECT id FROM states WHERE typeId in ($in) ORDER BY id;";
my $query = $dbh->prepare($sqlStr);
$query->execute(#values);
Note that if you use an ORM such as DBIx::Class instead, this sort of ugly hack gets abstracted away.
You have to build the SQL statement with the correct number of question marks and then set the parameter values. There is no way to bind a list to a single question mark.
I remember having problem with DBI method selectrow_array. When i wasn't tidy enough i got back from it not the value of the column i asked, but count of columns (or something unwanted, i can't recall exactly). Now i try to refactor some code and i want to make sure in every possible place, that i get back only expected value. So i try to avoid surprises and find out which the bad behaviour was. From DBI docs i read that this may be really be problematic situation:
If called in a scalar context for a statement handle that has more
than one column, it is undefined whether the driver will return the
value of the first column or the last. So don't do that. Also, in a
scalar context, an "undef" is returned if there are no more rows or if
an error occurred. That "undef" can't be distinguished from an "undef"
returned because the first field value was NULL. For these reasons
you should exercise some caution if you use "selectrow_array" in a
scalar context, or just don't do that.
Still i can't force selectrow_array to return anything but value of the col1 (that's it what i am expecting)
my $query = 'SELECT col1, col2, col3 FROM table WHERE id = 112233';
my ( $c ) = ( $dbh->selectrow_array( $query ) );
my $x = ask_from_db();
my $y = $dbh->selectrow_array( $query );
my $z = ( $dbh->selectrow_array( $query ) );
my #A = $dbh->selectrow_array( $query );
say "C: $c"; # C: col1
say "X: $x"; # X: col1
say "Y: $y"; # Y: col1
say "Z: $z"; # Z: col1
say "A: #A"; # A: col1 col2 col3
sub ask_from_db {
return $dbh->selectrow_array( $query );
}
Every way i ask above, gives me fine result. How should i run the query to get wrong result?
wrong result != col1 value
The difference in outcome will be based on the implementation of the driver.
wantarray ? #row : $row[0]
vs
wantarray ? #row : $row[-1]
You'd use to use a different driver to get a different outcome. That said, I imagine you'll have a hard time finding a driver that doesn't return the first.
If you want to be sure to get the first, use:
( $dbh->selectrow_array( $query ) )[0]
What the documentation means by "it is undefined whether the driver will return the value of the first column or the last" is that the column returned is defined by the database driver and not DBI.
So the Postgres driver may decide to always return the first column whereas the mysql driver may always return the last column, or the column returned might depend on the query.
So don't call selectrow_array is scalar context - always call it in list context:
my #row = $sth->selectrow_array($query)
and you'll avoid all of the issues that the documentation mentions.
I am writing small snippets in Perl and DBI (SQLite yay!)
I would like to log some specific queries to text files having the same filename as that of the table name(s) on which the query is run.
Here is the code I use to dump results to a text file :
sub dumpResultsToFile {
my ( $query ) = #_;
# Prepare and execute the query
my $sth = $dbh->prepare( $query );
$sth->execute();
# Open the output file
open FILE, ">results.txt" or die "Can't open results output file: $!";
# Dump the formatted results to the file
$sth->dump_results( 80, "\n", ", ", \*FILE );
# Close the output file
close FILE or die "Error closing result file: $!\n";
}
Here is how I can call this :
dumpResultsToFile ( <<" END_SQL" );
SELECT TADA.fileName, TADA.labelName
FROM TADA
END_SQL
What I effectively want is, instead of stuff going to "results.txt" ( that is hardcoded above ), it should now go to "TADA.txt".
Had this been a join between tables "HAI" and "LOL", then the resultset should be written to "HAI.LOL.txt"
Is what I am saying even possible using some magic in DBI?
I would rather do without parsing the SQL query for tables, but if there is a widely used and debugged function to grab source table names in a SQL query, that would work for me too.
What I want is just to have a filename
that gives some hint as to what query
output it holds. Seggregating based on
table name seems a nice way for now.
Probably not. Your SQL generation code takes the wrong approach. You are hiding too much information from your program. At some point, your program knows which table to select from. Instead of throwing that information away and embedding it inside an opaque SQL command, you should keep it around. Then your logger function doesn't have to guess where the log data should go; it knows.
Maybe this is clearer with some code. Your code looks like:
sub make_query {
my ($table, $columns, $conditions) = #_;
return "SELECT $columns FROM $table WHERE $conditions";
}
sub run_query {
my ($query) = #_;
$dbh->prepare($query);
...
}
run_query( make_query( 'foo', '*', '1=1' ) );
This doesn't let you do what you want to do. So you should structure
your program to do something like:
sub make_query {
my ($table, $columns, $conditions) = #_;
return +{
query => "SELECT $columns FROM $table WHERE $conditions",
table => $table,
} # an object might not be a bad idea
}
sub run_query {
my ($query) = #_;
$dbh->prepare($query->{query});
log_to_file( $query->{table}.'.log', ... );
...
}
run_query( make_query( 'foo', '*', '1=1' ) );
The API is the same, but now you have the information you need to log
the way you want.
Also, consider SQL::Abstract for dynamic SQL generation. My code
above is just an example.
Edit: OK, so you say you're using SQLite. It has an EXPLAIN command
which you could parse the output of:
sqlite> explain select * from test;
0|Trace|0|0|0|explain select * from test;|00|
1|Goto|0|11|0||00|
2|SetNumColumns|0|2|0||00|
3|OpenRead|0|2|0||00|
4|Rewind|0|9|0||00|
5|Column|0|0|1||00|
6|Column|0|1|2||00|
7|ResultRow|1|2|0||00|
8|Next|0|5|0||00|
9|Close|0|0|0||00|
10|Halt|0|0|0||00|
11|Transaction|0|0|0||00|
12|VerifyCookie|0|1|0||00|
13|TableLock|0|2|0|test|00|
14|Goto|0|2|0||00|
Looks like TableLock is what you would want to look for. YMMV, this
is a bad idea.
In general, in SQL, you cannot reliably deduce table names from result set, both for theoretical reasons (the result set may only consist of computed columns) and practical (the result set never includes table names - only column names - in its data).
So the only way to figure out the tables used is to stored them with (or deduce them from) the original query.
I've heard good things about the parsing ability of SQL::Statement but never used it before now myself.
use SQL::Statement;
use strict;
use warnings;
my $sql = <<" END_SQL";
SELECT TADA.fileName, TADA.labelName
FROM TADA
END_SQL
my $parser = SQL::Parser->new();
$parser->{RaiseError} = 1;
$parser->{PrintError} = 0;
my $stmt = eval { SQL::Statement->new($sql, $parser) }
or die "parse error: $#";
print join',',map{$_->name}$stmt->tables;
I need to insert values in database using Perl's DBI module. I have parsed a file to obtain these values and hence these values are present in an arrays, say #array1, #array2, #array3. I know how to insert one value at a time but not from an arrays.
I know insert one value at a time:
$dbh = DBI->connect("dbi:Sybase:server=$Srv;database=$Db", "$user", "$passwd") or die "could not connect to database";
$query= "INSERT INTO table1 (id, name, address) VALUES (DEFAULT, tom, Park_Road)";
$sth = $dbh->prepare($query) or die "could not prepare statement\n";
$sth-> execute or die "could not execute statement\n $command\n";
I am not sure if I have array1 containing ids, array2 containing names, and array3 containing address, how would I insert values.
Since you have parallel arrays, you could take advantange of execute_array:
my $sth = $dbh->prepare('INSERT INTO table1 (id, name, address) VALUES (?, ?, ?)');
my $num_tuples_executed = $sth->execute_array(
{ ArrayTupleStatus => \my #tuple_status },
\#ids,
\#names,
\#addresses,
);
Please note that this is a truncated (and slightly modified) example from the documentation. You'll definitely want to check out the rest of it if you decide to use this function.
Use placeholders.
Update: I just realized you have parallel arrays. That is really not a good way of working with data items that go together. With that caveat, you can use List::MoreUtils::each_array:
#!/usr/bin/perl
use strict; use warnings;
use DBI;
use List::MoreUtils qw( each_array );
my $dbh = DBI->connect(
"dbi:Sybase:server=$Srv;database=$Db",
$user, $passwd,
) or die sprintf 'Could not connect to database: %s', DBI->errstr;
my $sth = $dbh->prepare(
'INSERT INTO table1 (id, name, address) VALUES (?, ?, ?)'
) or die sprintf 'Could not prepare statement: %s', $dbh->errstr;
my #ids = qw( a b c);
my #names = qw( d e f );
my #addresses = qw( g h i);
my $it = each_array(#ids, #names, #address);
while ( my #data = $it->() ) {
$sth->execute( #data )
or die sprintf 'Could not execute statement: %s', $sth->errstr;
}
$dbh->commit
or die sprintf 'Could not commit updates: %s', $dbh->errstr;
$dbh->disconnect;
Note that the code is not tested.
You might also want to read the FAQ: What's wrong with always quoting "$vars"?.
Further, given that the only way you are handling error is by dying, you might want to consider specifying { RaiseError => 1 } in the connect call.
How could you not be sure what your arrays contain? Anyway the approach would be the iterate through one array and assuming the other arrays have corresponding values put those into the insert statement
Another way would be to use a hash as an intermediate storage area. IE:
my $hash = {};
foreach(#array1) {
$hash->{id} = $array1[$_];
$hash->{name} = $array2[$_];
$hash->{address} = $array3[$_];
}
foreach( keys %$hash ) {
$sql = "insert into table values(?,?,?)";
$sth = $dbh->prepare($sql) or die;
$sth->execute($hash->{id}, $hash->{name}, $hash->{address}) or die;
}
Though again this depends on the three arrays being synced up. However you could modify this to do value modifications or checks or greps in the other arrays within the first loop through array1 (ie: if your values in array2 and array3 are maybe stored as "NN-name" and "NN-address" where NN is the id from the first array and you need to find the corresponding values and remove the NN- with a s// regex). Depends on how your data is structured though.
Another note is to check out Class::DBI and see if it might provide a nicer and more object oriented way of getting your data in.