如果有人帮我，谢谢你的预付款。
以下是根据不同参数筛选某些客户的查询。在最后一个筛选器中，我需要根据查询中的计算字段对客户进行筛选，当我尝试在where子句中使用计算字段时，它会显示未知字段的错误，我应该怎么做？

$sql = <<<SQL

        SELECT c.*, t.name as town_name, r.name as region_name, IFNULL((
            select sum(p.remaining) 
            from payments p
            left join sales s
            on s.payment_id = p.id
            where s.customer_id = c.id and s.status = 1
        ), 0) as credit_sales, 
        (
            IFNULL((
                SELECT sum(cl.amount)
                FROM customer_ledgers cl
                WHERE cl.customer_id = c.id and cl.status = 1 and cl.type = 1
            ), 0) 
            -
            IFNULL((
                SELECT sum(amount)
                FROM customer_ledgers
                WHERE customer_id = c.id and status=1 and type = 2
            ), 0)
        ) as balancex, (
          IFNULL( (
            SELECT `amount`
            FROM `customer_ledgers`
            WHERE customer_id = c.id and status = 1
            ORDER BY `date` DESC 
            LIMIT 1
          ), 0)
        ) as last_amount

        from customers c
        left join regions r
        on r.id = c.region_id
        left join towns t
        on  t.id = c.town_id

sql语句

# we need where?

    $sql .= 'WHERE c.status = 1';
    if (isset($params['CustomerSearch'])) {
        $sql .= " and ";
    }

    # add where statements to the query

    if (isset($params['CustomerSearch'])) {
            $isname     = isset($params['CustomerSearch']['name']);
            $istown     = isset($params['CustomerSearch']['region_name']);
            $isregion   = isset($params['CustomerSearch']['town_name']);
            $isbalance   = isset($params['CustomerSearch']['balancex']);

        if (isset($params['CustomerSearch']['name'])) {
            $this->name = trim($params['CustomerSearch']['name']);
            $sql .= " c.name like '%{$this->name}%'";
        }

        if (isset($params['CustomerSearch']['region_name']) && $params['CustomerSearch']['region_name'] !== '') {
            $this->region_name = trim($params['CustomerSearch']['region_name']);

            if ($isname) {
                $sql .= " and ";
            }
            $sql .= " r.id=" . $params['CustomerSearch']['region_name'];
        }

        if (isset($params['CustomerSearch']['town_name']) && $params['CustomerSearch']['town_name'] !== '') {
               $this->town_name = trim($params['CustomerSearch']['town_name']);

               if ($isname || $isregion) {
                   $sql .= " AND t.id = {$this->town_name}";
               }
           } 

        if (isset($params['CustomerSearch']['balancex']) && $params['CustomerSearch']['balancex'] !== '') {
               $this->balancex = trim($params['CustomerSearch']['balancex']);

               if ($isbalance) {
                   $sql .= " AND balancex > {$isbalance}";
               }
           }   
    }

    # town where clause.

    $result = Yii::$app->db->createCommand($sql)->queryAll();

    return $result;