Ask the database exactly the question you want answered.

A query is a question for the database. SELECT name, age FROM pets WHERE age > 2 ORDER BY age DESC reads almost like English: these columns, from this table, only rows that match, biggest age first. Without DESC, ORDER BY goes smallest first.

fetchAll(PDO::FETCH_ASSOC) collects every row of the answer into an array of records, like the nested arrays in unit 1. PDO::FETCH_ASSOC is a constant that belongs to PDO, reached with ::. It asks for rows with keys.

fetch(PDO::FETCH_ASSOC) hands you one row at a time, and false when none are left. It suits questions with one answer, like a count.

<?php
$db = new PDO("sqlite::memory:");
$db->exec("CREATE TABLE pets (name TEXT, age INTEGER)");
$db->exec("INSERT INTO pets VALUES ('Rex', 3), ('Tilly', 5), ('Pip', 1)");
$rows = $db->query("SELECT name, age FROM pets WHERE age > 2 ORDER BY age DESC")->fetchAll(PDO::FETCH_ASSOC);
foreach ($rows as $pet) {
    echo "{$pet["name"]}: {$pet["age"]}\n";
}
$count = $db->query("SELECT COUNT(*) AS total FROM pets")->fetch(PDO::FETCH_ASSOC);
echo "{$count["total"]} pets in all\n";

COUNT(*) counts rows, and AS total gives that answer a key you can use.