diff --git a/phpBB/phpbb/db/driver/driver.php b/phpBB/phpbb/db/driver/driver.php
index 2925765e94..e497e6dda1 100644
--- a/phpBB/phpbb/db/driver/driver.php
+++ b/phpBB/phpbb/db/driver/driver.php
@@ -774,7 +774,18 @@ abstract class driver implements driver_interface
 
 				if (!empty($array['WHERE']))
 				{
-					$sql .= ' WHERE ' . $this->_sql_custom_build('WHERE', $array['WHERE']);
+					$sql .= ' WHERE ';
+
+					if (is_array($array['WHERE']))
+					{
+						$sql_where = $this->_process_boolean_tree_first($array['WHERE']);
+					}
+					else
+					{
+						$sql_where = $array['WHERE'];
+					}
+
+					$sql .= $this->_sql_custom_build('WHERE', $sql_where);
 				}
 
 				if (!empty($array['GROUP_BY']))
@@ -793,6 +804,130 @@ abstract class driver implements driver_interface
 		return $sql;
 	}
 
+
+	protected function _process_boolean_tree_first($operations_ary)
+	{
+		// In cases where an array exists but there is no head condition,
+		// it should be because there's only 1 WHERE clause. This seems the best way to deal with it.
+		if ($operations_ary[0] !== 'AND' &&
+			$operations_ary[0] !== 'OR')
+		{
+			$operations_ary = array('AND', $operations_ary);
+		}
+		return $this->_process_boolean_tree($operations_ary) . "\n";
+	}
+
+	protected function _process_boolean_tree($operations_ary)
+	{
+		$operation = array_shift($operations_ary);
+
+		foreach ($operations_ary as &$condition)
+		{
+			switch ($condition[0])
+			{
+				case 'AND':
+				case 'OR':
+
+					$condition = ' ( ' . $this->_process_boolean_tree($condition) . ') ';
+
+				break;
+				case 'NOT':
+
+					$condition = ' NOT (' . $this->_process_boolean_tree($condition) . ') ';
+
+				break;
+
+				default:
+
+					switch (sizeof($condition))
+					{
+						case 3:
+
+							// Typical 3 element clause with {left hand} {operator} {right hand}
+							switch ($condition[1])
+							{
+								case 'IN':
+								case 'NOT_IN':
+
+									// As this is used with an IN, assume it is a set of elements for sql_in_set()
+									$condition = $this->sql_in_set($condition[0], $condition[2], $condition[1] === 'NOT_IN', true);
+
+								break;
+
+								case 'LIKE':
+
+									$condition = $condition[0] . ' ' . $this->sql_like_expression($condition[2]) . ' ';
+
+								break;
+
+								case 'NOT_LIKE':
+
+									$condition = $condition[0] . ' ' . $this->sql_not_like_expression($condition[2]) . ' ';
+
+								break;
+
+								case 'IS_NOT':
+
+									$condition[1] = 'IS NOT';
+
+								// no break
+								case 'IS':
+
+									// If the value is NULL, the string of it is the empty string ('') which is not the intended result.
+									// this should solve that
+									if ($condition[2] === null)
+									{
+										$condition[2] = 'NULL';
+									}
+
+									$condition = implode(' ', $condition);
+
+								break;
+
+								default:
+
+									$condition = implode(' ', $condition);
+
+								break;
+							}
+
+						break;
+
+						case 5:
+
+							// Subquery with {left hand} {operator} {compare kind} {SELECT Kind } {Sub Query}
+
+							$condition = $condition[0] . ' ' . $condition[1] . ' ' . $condition[2] . ' ( ';
+							$condition .= $this->sql_build_query($condition[3], $condition[4]);
+							$condition .= ' )';
+
+						break;
+
+						default:
+							// This is an unpredicted clause setup. Just join all elements.
+							$condition = implode(' ', $condition);
+
+						break;
+					}
+
+				break;
+			}
+
+		}
+
+		if($operation === 'NOT')
+		{
+			$operations_ary =  implode("", $operations_ary);
+		}
+		else
+		{
+			$operations_ary = implode(" \n	$operation ", $operations_ary);
+		}
+
+		return $operations_ary;
+	}
+
+
 	/**
 	* {@inheritDoc}
 	*/
diff --git a/tests/dbal/boolean_processor_test.php b/tests/dbal/boolean_processor_test.php
new file mode 100644
index 0000000000..8662485ac8
--- /dev/null
+++ b/tests/dbal/boolean_processor_test.php
@@ -0,0 +1,306 @@
+<?php
+/**
+*
+* This file is part of the phpBB Forum Software package.
+*
+* @copyright (c) phpBB Limited <https://www.phpbb.com>
+* @license GNU General Public License, version 2 (GPL-2.0)
+*
+* For full copyright and license information, please see
+* the docs/CREDITS.txt file.
+*
+*/
+
+require_once dirname(__FILE__) . '/../../phpBB/includes/functions.php';
+require_once dirname(__FILE__) . '/../../phpBB/includes/utf/utf_tools.php';
+
+class phpbb_boolean_processor_test extends phpbb_database_test_case
+{
+	public function getDataSet()
+	{
+		return $this->createXMLDataSet(dirname(__FILE__).'/fixtures/boolean_processor.xml');
+	}
+
+	public function test_single_not_like()
+	{
+		$db = $this->new_dbal();
+
+		$db->sql_return_on_error(true);
+
+		$sql_ary = array(
+			'SELECT'	=> 'u.user_id',
+			'FROM'		=> array(
+				'phpbb_users'		=> 'u',
+			),
+			'WHERE'		=> array('u.username_clean', 'NOT_LIKE', 'gr' . $db->get_any_char()),
+			'ORDER_BY'	=> 'u.user_id',
+		);
+		$sql = $db->sql_build_query('SELECT', $sql_ary);
+		$result = $db->sql_query($sql);
+
+		$db->sql_return_on_error(false);
+
+		$this->assertEquals(array(
+			array('user_id' => '1'),
+			array('user_id' => '2'),
+			array('user_id' => '3'),
+			array('user_id' => '6'),
+			), $db->sql_fetchrowset($result),
+				($result === false) ?
+				"SQL ERROR:<br>" . var_export($sql, true) . "<br>" . $db->sql_error() :
+				var_export($sql, true) . '   ' . var_export($result, true)
+			);
+	}
+
+	public function test_single_like()
+	{
+		$db = $this->new_dbal();
+
+		$db->sql_return_on_error(true);
+
+		$sql_ary = array(
+			'SELECT'	=> 'u.user_id',
+			'FROM'		=> array(
+				'phpbb_users'		=> 'u',
+			),
+			'WHERE'		=> array('u.username_clean', 'LIKE', 'gr' . $db->get_any_char()),
+			'ORDER_BY'	=> 'u.user_id',
+		);
+		$sql = $db->sql_build_query('SELECT', $sql_ary);
+		$result = $db->sql_query($sql);
+
+		$db->sql_return_on_error(false);
+
+		$this->assertEquals(array(
+			array('user_id' => '4'),
+			array('user_id' => '5'),
+			), $db->sql_fetchrowset($result),
+				($result === false) ?
+				"SQL ERROR:<br>" . var_export($sql, true) . "<br>" . $db->sql_error() :
+				var_export($sql, true) . '   ' . var_export($result, true)
+			);
+	}
+
+	public function test_single_not_in()
+	{
+		$db = $this->new_dbal();
+
+		$db->sql_return_on_error(true);
+
+		$sql_ary = array(
+			'SELECT'	=> 'u.user_id',
+			'FROM'		=> array(
+				'phpbb_users'		=> 'u',
+			),
+			'WHERE'		=> array('u.user_id', 'NOT_IN', array(3,4,5)),
+			'ORDER_BY'	=> 'u.user_id',
+		);
+		$sql = $db->sql_build_query('SELECT', $sql_ary);
+		$result = $db->sql_query($sql);
+
+		$db->sql_return_on_error(false);
+
+		$this->assertEquals(array(
+			array('user_id' => '1'),
+			array('user_id' => '2'),
+			array('user_id' => '6'),
+			), $db->sql_fetchrowset($result),
+				($result === false) ?
+				"SQL ERROR:<br>" . var_export($sql, true) . "<br>" . $db->sql_error() :
+				var_export($sql, true) . '   ' . var_export($result, true)
+			);
+	}
+
+	public function test_single_in()
+	{
+		$db = $this->new_dbal();
+
+		$db->sql_return_on_error(true);
+
+		$sql_ary = array(
+			'SELECT'	=> 'u.user_id',
+			'FROM'		=> array(
+				'phpbb_users'		=> 'u',
+			),
+			'WHERE'		=> array('u.user_id', 'IN', array(3,4,5)),
+			'ORDER_BY'	=> 'u.user_id',
+		);
+		$sql = $db->sql_build_query('SELECT', $sql_ary);
+		$result = $db->sql_query($sql);
+
+		$db->sql_return_on_error(false);
+
+		$this->assertEquals(array(
+			array('user_id' => '3'),
+			array('user_id' => '4'),
+			array('user_id' => '5'),
+			), $db->sql_fetchrowset($result),
+				($result === false) ?
+				"SQL ERROR:<br>" . var_export($sql, true) . "<br>" . $db->sql_error() :
+				var_export($sql, true) . '   ' . var_export($result, true)
+			);
+	}
+
+	public function test_and_of_or_of_and()
+	{
+		$db = $this->new_dbal();
+
+		$db->sql_return_on_error(true);
+
+		$sql_ary = array(
+			'SELECT'	=> 'u.user_id',
+			'FROM'		=> array(
+				'phpbb_users'		=> 'u',
+				'phpbb_user_group'	=> 'ug',
+			),
+			'LEFT_JOIN'	=> array(
+				array(
+					'FROM'	=> array(
+						'phpbb_banlist'	=> 'b',
+					),
+					'ON'	=> 'u.user_id = b.ban_userid',
+				),
+			),
+			'WHERE'		=> array('AND',
+				array('OR',
+					array('AND',
+						array('ug.user_id', 'IN', array(1, 2, 3, 4)),
+						array('ug.group_id', '=', 2),
+					),
+					array('AND',
+						array('ug.group_id', '=', 1),
+						array('b.ban_id', 'IS_NOT', NULL),
+					),
+				),
+				array('u.user_id', '=', 'ug.user_id'),
+			),
+			'ORDER_BY'	=> 'u.user_id',
+		);
+		$sql = $db->sql_build_query('SELECT', $sql_ary);
+		$result = $db->sql_query($sql);
+
+		$db->sql_return_on_error(false);
+
+		$this->assertEquals(array(
+			array('user_id' => '2'),
+			array('user_id' => '4'),
+			), $db->sql_fetchrowset($result),
+				($result === false) ?
+				"SQL ERROR:<br>" . var_export($sql, true) . "<br>" . $db->sql_error() :
+				var_export($sql, true) . '   ' . var_export($result, true)
+			);
+	}
+
+	public function test_triple_and_with_in()
+	{
+		$db = $this->new_dbal();
+
+		$db->sql_return_on_error(true);
+
+		$sql_ary = array(
+			'SELECT'	=> 'u.user_id',
+			'FROM'		=> array(
+				'phpbb_users'		=> 'u',
+				'phpbb_user_group'	=> 'ug',
+			),
+			'WHERE'		=> array('AND',
+				array('ug.user_id', 'IN', array(1, 2, 3, 4)),
+				array('ug.group_id', '=', 1),
+				array('u.user_id', '=', 'ug.user_id'),
+			),
+			'ORDER_BY'	=> 'u.user_id',
+		);
+		$sql = $db->sql_build_query('SELECT', $sql_ary);
+		$result = $db->sql_query($sql);
+
+		$db->sql_return_on_error(false);
+
+		$this->assertEquals(array(
+			array('user_id' => '1'),
+			array('user_id' => '2'),
+			array('user_id' => '3'),
+			), $db->sql_fetchrowset($result),
+			($result === false) ?
+				"SQL ERROR:<br>" . var_export($sql, true) . "<br>" . $db->sql_error() :
+				var_export($sql, true) . '   ' . var_export($result, true)
+		);
+
+	}
+
+	public function test_double_and_with_not_of_or()
+	{
+		$db = $this->new_dbal();
+
+		$db->sql_return_on_error(true);
+
+		$sql_ary = array(
+			'SELECT'	=> 'u.user_id',
+			'FROM'		=> array(
+				'phpbb_users'		=> 'u',
+				'phpbb_user_group'	=> 'ug',
+			),
+			'WHERE'		=> array('AND',
+				array('NOT',
+					array('OR',
+						array('ug.group_id', '=', 1),
+						array('ug.group_id', '=', 2),
+					),
+				),
+				array('u.user_id', '=', 'ug.user_id'),
+			),
+			'ORDER_BY'	=> 'u.user_id',
+		);
+		$sql = $db->sql_build_query('SELECT', $sql_ary);
+		$result = $db->sql_query($sql);
+
+		$db->sql_return_on_error(false);
+
+		$this->assertEquals(array(), $db->sql_fetchrowset($result),
+				($result === false) ?
+				"SQL ERROR:<br>" . var_export($sql, true) . "<br>" . $db->sql_error() :
+				var_export($sql, true) . '   ' . var_export($result, true)
+			);
+	}
+
+	public function test_triple_and_with_is_null()
+	{
+		$db = $this->new_dbal();
+
+		$db->sql_return_on_error(true);
+
+		$sql_ary = array(
+			'SELECT'	=> 'u.username',
+			'FROM'		=> array(
+				'phpbb_users'		=> 'u',
+				'phpbb_user_group'	=> 'ug',
+			),
+			'LEFT_JOIN'	=> array(
+				array(
+					'FROM'	=> array(
+						'phpbb_banlist'	=> 'b',
+					),
+					'ON'	=> 'u.user_id = b.ban_userid',
+				),
+			),
+			'WHERE'		=> array('AND',
+				array('ug.group_id', '=', 1),
+				array('u.user_id', '=', 'ug.user_id'),
+				array('b.ban_id', 'IS', NULL),
+			),
+			'ORDER_BY'	=> 'u.username',
+		);
+		$sql = $db->sql_build_query('SELECT', $sql_ary);
+		$result = $db->sql_query($sql);
+
+		$db->sql_return_on_error(false);
+
+		$this->assertEquals(array(
+			array('username' => 'helper'),
+			array('username' => 'mass email'),
+			), $db->sql_fetchrowset($result),
+				($result === false) ?
+				"SQL ERROR:<br>" . var_export($sql, true) . "<br>" . $db->sql_error() :
+				var_export($sql, true) . '   ' . var_export($result, true)
+			);
+	}
+}
diff --git a/tests/dbal/fixtures/boolean_processor.xml b/tests/dbal/fixtures/boolean_processor.xml
new file mode 100644
index 0000000000..c5da677116
--- /dev/null
+++ b/tests/dbal/fixtures/boolean_processor.xml
@@ -0,0 +1,84 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<dataset>
+	<table name="phpbb_banlist">
+		<column>ban_id</column>
+		<column>ban_userid</column>
+		<row>
+			<value>1</value>
+			<value>2</value>
+		</row>
+	</table>
+	<table name="phpbb_users">
+		<column>user_id</column>
+		<column>username</column>
+		<column>username_clean</column>
+		<column>user_permissions</column>
+		<column>user_sig</column>
+		<row>
+			<value>1</value>
+			<value>mass email</value>
+			<value>mass email</value>
+			<value></value>
+			<value></value>
+		</row>
+		<row>
+			<value>2</value>
+			<value>banned</value>
+			<value>banned</value>
+			<value></value>
+			<value></value>
+		</row>
+		<row>
+			<value>3</value>
+			<value>helper</value>
+			<value>helper</value>
+			<value></value>
+			<value></value>
+		</row>
+		<row>
+			<value>4</value>
+			<value>GroupBPal</value>
+			<value>groupbpal</value>
+			<value></value>
+			<value></value>
+		</row>
+		<row>
+			<value>5</value>
+			<value>GroupBPal2</value>
+			<value>groupBPal2</value>
+			<value></value>
+			<value></value>
+		</row>
+		<row>
+			<value>6</value>
+			<value>not in group</value>
+			<value>not in group</value>
+			<value></value>
+			<value></value>
+		</row>
+	</table>
+	<table name="phpbb_user_group">
+		<column>user_id</column>
+		<column>group_id</column>
+		<row>
+			<value>1</value>
+			<value>1</value>
+		</row>
+		<row>
+			<value>2</value>
+			<value>1</value>
+		</row>
+		<row>
+			<value>3</value>
+			<value>1</value>
+		</row>
+		<row>
+			<value>4</value>
+			<value>2</value>
+		</row>
+		<row>
+			<value>5</value>
+			<value>2</value>
+		</row>
+	</table>
+</dataset>