* GROUP BY and HAVING clauses can be used for dealing with aggregate functions
* Following aggregate functions are available on DQL: COUNT, MAX, MIN, AVG, SUM

Selecting alphabetically first user by name.

<code type="sql">
SELECT MIN(u.name) FROM User u
</code>

Selecting the sum of all Account amounts.

<code type="sql">
SELECT SUM(a.amount) FROM Account a
</code>

* Using an aggregate function in a statement containing no GROUP BY clause, results in grouping on all rows. In the example above we fetch all users and the number of phonenumbers they have.

<code type="sql">
SELECT u.*, COUNT(p.id) FROM User u, u.Phonenumber p GROUP BY u.id
</code>

* The HAVING clause can be used for narrowing the results using aggregate values. In the following example we fetch all users which have atleast 2 phonenumbers

<code type="sql">
SELECT u.* FROM User u, u.Phonenumber p HAVING COUNT(p.id) >= 2
</code>

<code type="php">
// retrieve all users and the phonenumber count for each user

$users = $conn->query("SELECT u.*, COUNT(p.id) count FROM User u, u.Phonenumber p GROUP BY u.id");

foreach($users as $user) {
    print $user->name . ' has ' . $user->Phonenumber[0]->count . ' phonenumbers';
}
</code>