// SELECT u.name, r.name as role FROM t_users as u INNER JOIN t_roles as r ON u.role_id = r.id WHERE u.id = 123;
$user = $db->select('users as u', 'u.name, r.name as role')->join('roles as r', 'u.role_id = r.id')->where('u.id = 123')->fetch();

<?php

class DBQuery {
    private $_query,
            $_db;
    
    public function __construct($db) {
        $this->_db = $db;
    }
    
    public function select($table, $columns = '*') {
        $this->_query = 'SELECT ' . $columns . ' FROM ' . $this->_db->getTablename($table);
        return $this;
    }
    
    private function _prepare($data) {      
        foreach ($data as &$v) {
            if (is_null($v)) {
                $v = 'NULL';
            }
            else if (is_bool($v)) {
                $v = $v ? 'TRUE' : 'FALSE';
            }
            else if (is_string($v)) {
                $v = $this->_db->quote($v);
            }
        }
        
        return $data;
    }
    
    public function insert($table, $row) {
        $row = $this->_prepare($row);
        $this->_query = 'INSERT ' . $this->_db->getTablename($table);
        $this->_query .= ' (' . implode(', ', array_keys($row)) . ')';
        $this->_query .= ' VALUES (' . implode(', ', array_values($row)) . ')';
    }
    
    public function update($table, $set) {
        $set = $this->_prepare($set);
        $this->_query = 'UPDATE ' . $this->_db->getTablename($table) . ' SET';
        
        foreach ($set as $k => $v) {
            $this->_query .= " $k = $v,";
        }
        
        $this->_query = substr($this->_query, 0, -1);
        return $this;
    }
    
    public function delete($table) {
        $this->_query = 'DELETE ' . $this->_db->getTablename($table);
        return $this;
    }
    
    public function join($table, $condition, $type = 'INNER') {
        $this->_query .= ' ' . $type . ' JOIN ' . $this->_db->getTablename($table) . ' ON ' . $condition;
        return $this;
    }
    
    public function order($condition) {
        $this->_query .= " ORDER BY $condition";
        return $this;
    }
    
    public function group($group) {
        $this->_query .= " GROUP BY $condition";
        return $this;
    }
    
    public function where($condition) {
        $this->_query .= " WHERE $condition";
        return $this;
    }
    
    public function limit($offset, $limit = null) {
        $offset = (int) $offset;
        $this->_query .= ' LIMIT ' . (!is_null($limit) ? $offset . ', ' . intval($limit) : $offset);
        return $this;
    }
    
    public function __toString() {
        return $this->_query . ';';
    }
    
    public function query() {
        return $this->_db->query($this);
    }
    
    public function fetch() {
        $result = $this->query();
        return $this->_db->fetch($result);
    }
    
    public function fetchAll() {
        $result = $this->query();
        return $this->_db->fetchAll($result);
    }
}

?>

    public function select($table, $columns) {
        $query = new DBQuery($this);
        $query->select($table, $columns);
        return $query;
    }

    public function get($table, $columns = '*', $where = null) {
        $select = $this->select($table, $columns);
        
        if ($where) {
            $select->where($where);
        }
        
        return $select->fetch();
    }
    
    public function find($table, $id, $columns = '*') {
        return $this->get($table, $columns, 'id = ' . intval($id));
    }

    public function buildQuery() {
        return new DBQuery($this);
    }
    
    public function select($columns = '*', $table = null) {
        $query = $this->buildQuery();
        $query->select($columns, $table);
        return $query;
    }

    // ...
    
    public function getAll($table, $columns = '*', $where = null, $offset = null, $limit = null, $order = null, $group = null) {
        $select = $this->select($columns, $table);
        
        if ($where) {
            $select->where($where);
        }
        
        if ($order) {
            $select->order($order);
        }
        
        if ($group) {
            $select->group($group);
        }
        
        if ($offset) {
            $select->limit($offset, $limit);
        }
         
        return $select->fetchAll();
    }

    public function fetchOne($table, $column, $where = null) {
        $query = $this->select($column, $table)->where($where);
        return $this->one($query);
    }

$v = $db->fetchOne($table, '(max(field)+min(field))/count(field)', $where);

$param = array('bla %i bla bla %s bla %i bla bla', 'foo', 'bar', '10');
echo($param[0]) . '<br>';

$parts = explode('%', $param[0]);

$i = 0;
$j = count($parts);

echo $parts[0];

while (++$i < $j) {
    switch ($parts[$i][0]) {
        case 'i':
            echo(intval($param[$i]));
            break;
        
        case 's':
            echo("'" . $param[$i] . "'");
            break;
    }
    
    echo substr($parts[$i], 1);
}

echo '<br>';

bla %i bla bla %s bla %i bla bla
bla 0 bla bla 'bar' bla 10 bla bla

    public function where($param) {       
        $this->_query .= ' WHERE ';
        
        if (is_array($param)) {
            if (count($param) > 1) {
                $parts = explode('%', $param[0]);
                $this->_query .= $parts[0];
                $i = 0;
                $j = count($parts);

                while (++$i < $j) {
                    switch ($parts[$i][0]) {
                        case 'i':
                            $this->_query .= intval($param[$i]);
                            break;

                        case 's':
                            $this->_query .= $this->_db->quote($param[$i]);
                            break;
                    }

                    $this->_query .= substr($parts[$i], 1);
                }
            } 
            else {
                $this->_query .= $param[0];
            }
        }
        else {
            $this->_query .= $param;
        }
        
        return $this;
    }

$user = $db->get('users', array('name = %s AND pass = %s', $_POST['name'], $_POST['pass']));

    public function where($param) {       
        $this->_query .= ' WHERE ';
        
        if (is_array($param)) {
            $i = count($param);
            
            if ($i > 1) {
                $parts = explode('%', $param[0]);
                $j = count($parts);
                
                if ($i != $j) {
                    throw new Exception("Wrong number of modifiers.");
                }
                
                $this->_query .= $parts[0];
                $k = 0;

                while ($j > ++$k) {
                    switch ($parts[$k][0]) {
                        case 'i':
                            $this->_query .= intval($param[$k]);
                            break;

                        case 's':
                            $this->_query .= $this->_db->quote($param[$k]);
                            break;
                        
                        default:
                            throw new Exception("Unknown modifier '{$parts[$k][0]}'.");
                    }

                    $this->_query .= substr($parts[$k], 1);
                }
            } 
            else {
                $this->_query .= $param[0];
            }
        }
        else {
            $this->_query .= $param;
        }
        
        return $this;
    }