private function checkSubquery($expression): string
    {
        if (is_array($expression)) {
            $qb = new QueryBuilder($expression);
            
            // Build sql string and replace the existing expression with it
            $expression = '(' . $qb->build() . ')';
            
            // Add possible params to parents params array
            $this->params += $qb->getParams();
        }
        
        return $expression;
    }