framework/db/QueryExpressionBuilder.php
public function build(ExpressionInterface $expression, array &$params = [])
    {
        list($sql, $params) = $this->queryBuilder->build($expression, $params);

        return "($sql)";
    }
public function build(ExpressionInterface $expression, array &$params = [])
    {
        list($sql, $params) = $this->queryBuilder->build($expression, $params);

        return "($sql)";
    }
Method builds the raw SQL from the $expression that will not be additionally
escaped or quoted.

@param ExpressionInterface|Query $expression the expression to be built.
@param array $params the binding parameters.
@return string the raw SQL that will not be additionally escaped or quoted.