Perl database interface (DBI). We will learn just enough SQL to use Perl DBI to read/write to a SQLite database.

1. DBI

2. SQL dialects
SQLite
MySQL
PostgreSQL
Oracle

3. Case insensitive
SQL language is case insensitive
Textual data is case sensitive
SQL reserved words in CAPs
– By convention
– Not mandatory

4. SQLite
$ sqlite3 address.db

5. SQL Create
CREATE TABLE address (
id INTEGER PRIMARY KEY,
first_name TEXT,
last_name TEXT,
city TEXT,
state TEXT
);

6. SQLite
sqlite> .tables
sqlite> .schema address
sqlite> .quit

7. SQL Insert
INSERT INTO address
(first_name, last_name, city, state)
VALUES ('John', 'Doe', 'Hollywood', 'CA');

8. SQL Update
UPDATE address
SET city='Hollywood',
state='FL'
WHERE id=1;

9. SQL Select
SELECT * FROM address;
-- show everything
SELECT *
FROM address
WHERE state='FL';
-- only florida
SELECT first_name, last_name
FROM address
WHERE state='FL';
-- specific fields

10. SQL Delete
DELETE FROM address;
-- very dangerous
-- deletes everything
DELETE FROM address
WHERE state='FL';
-- use a WHERE condition

11. DBI
$ cpanm DBD::SQLite

12. DBI->connect()
#!/usr/bin/perl
use warnings;
use strict;
use DBI;
my $dbh = DBI->connect(
"dbi:SQLite:dbname=address.db", "", ""
);

13. prepare/execute/fetch
$sth = $dbh->prepare ("
SELECT id, first_name, last_name, city, state
FROM address
WHERE city=?
AND state=?
");
$sth->execute ('Sacramento', 'CA');
# fills in values to the ? placeholders
while (my $row = $sth->fetch()) {
my ($id, $first_name, $last_name, $city, $state) = @$row;
print "$city, $state: $first_name $last_namen";
}

14. Placeholder
?
Security: The placeholder escapes data
– Counters SQL injection attack
Optimization: Prepare once, execute many. Sometimes faster.
– MySQL: no difference
– Oracle: faster

15. fetchrow_hashref
while (my $row = $sth->fetchrow_hashref() ) {
print Dumper $row;
}
# $VAR1 = {
# 'city' => 'Sacramento',
# 'id' => 2,
# 'state' => 'CA',
# 'first_name' => 'Jane',
# 'last_name' => 'Doe'
# };

16. fetchall_arrayref()
# No WHERE condition. Get everything.
$sth = $dbh->prepare ("
SELECT id, first_name, last_name, city, state
FROM address
");
$sth->execute ();
my $rows = $sth->fetchall_arrayref();
foreach my $row (@$rows) {
my ($id, $first_name, $last_name, $city, $state) = @$row;
print "$city, $state: $first_name $last_namen";
}

17. No fetch
$sth = $dbh->prepare ("
INSERT INTO address (first_name, last_name, city, state)
VALUES (?,?,?,?)
");
$sth->execute('Doug', 'Engelbart', 'Moscow', 'Idaho');

18. No fetch
$sth = $dbh->prepare ("
UPDATE address SET state=?
WHERE city=?
");
$sth->execute ('TN', 'Moscow');

19. No fetch
$sth = $dbh->prepare ("
UPDATE address SET first_name=?, last_name=?
WHERE city=? AND state=?
");
$sth->execute ('Edgar', 'Codd', 'TN', 'Moscow');

20. No fetch
$sth = $dbh->prepare ("
DELETE FROM address
WHERE city=? AND state=?
");
$sth->execute ('Moscow', 'TN');

21. last_insert_id()
# after your INSERT statement
my $row_id = $dbh->last_insert_id(
undef, undef, undef, undef
);
print "row_id: $row_idn";

22. Caching the dbh
Opening a database connection incurs overhead.
You want to cache the database handle
– Open once.
– Keep it open.
Example of caching on next slide

23. state
use feature 'state'; # perl 5.10
sub get_dbh {
state $dbh;
if (!$dbh) {
$dbh = DBI->connect(
"dbi:SQLite:dbname=address.db", "", ""
);
}
return $dbh;
}