如何从 MySQL 中的比较和空检查指定 ORDER BY？

Question

我有两个 tables feed_old（默认数据）和 feed_new（新数据），其中 cron 作业将每天运行并且仅更新 feed_new table 与当前信息

Cron 作业

$url = "localhost/test.xml";
$xml = simplexml_load_file($url);

$this->db->query("TRUNCATE TABLE feed_new");

$date = date('Y-m-d H:i:s');

foreach($xml->Product as $item)
{
  $data = array(
    'product_code'    => $item->ProductCode,
    'name'            => $item->Name,
    'price'           => $item->RetailCurrentPrice,
    'stock'           => (int)$item->Stock,
    'manufacture'           => '1',
    'date_updated'      => $date
  );

  $update_status = $this->model_price->check_price($data);
}

型号

public function check_price($data)
{
  if($data) {
    $insert = $this->db->insert('feed_new', $data);
    return ($insert == true) ? true : false;
  }
}

自此一切正常

当我比较 feed_new 与 feed_old 并获取所有更改并显示所有记录时，问题就来了

比较 feed_new 与 feed_old 之后，我想从 table 中提取所有记录并按级别排序

级别 1 - 如果产品有不同的价格 feed_new.price <> feed_old.price

2 级 - 如果来自 feed_old 的产品在 feed_new 中不存在（这意味着产品不再受支持）

3 级 - 如果来自 feed_new 的产品在 feed_old 中不存在（这意味着产品是新的）

4 级 - 其余结果

比较查询

SELECT fn.name AS name_new, fo.date_updated, fo.id, fo.name,fo.price,fo.product_code, fn.product_code AS product_code_new, fo.stock, fn.price AS price_new, fn.stock AS stock_new, fn.date_updated AS date_updated_new 
FROM feed_old fo 
LEFT JOIN feed_new fn ON fo.product_code = fn.product_code 

UNION ALL 

SELECT fn.name AS name_new, fo.date_updated, fo.id, fo.name,fo.price,fo.product_code, fn.product_code AS product_code_new, fo.stock, fn.price AS price_new, fn.stock AS stock_new, fn.date_updated AS date_updated_new 
FROM feed_old fo 
RIGHT JOIN feed_new fn ON fn.product_code = fo.product_code 


WHERE fo.product_code IS NULL OR fn.name IS NULL ORDER BY COALESCE(price <> price_new, name_new is NULL, product_code IS NULL) DESC

问题是级别 - 3 总是显示在最后一条记录中，我的意思是在级别 - 4 之后在我下面的工作示例中，您可以看到 name_new -> test3 位于 table 的底部，而此时应该位于第 3 个位置

如何按以上级别订购它们

Working Example

Answer 1

COALESCE 似乎不是最适合此排序的函数，因为它只是 returns 第一个非 NULL 值，这在进行比较时不直观操作数可以是 NULL.

建议使用 CASE 而不是级别的虚拟整数值 - 如下所示：

ORDER BY CASE WHEN price_new IS NOT NULL AND price_new <> price THEN 1
              WHEN price_new IS NULL THEN 2
              WHEN price IS NULL THEN 3
              ELSE 4
         END

查看此 SQLFiddle 演示：http://sqlfiddle.com/#!9/57c8eb3/6。

Answer 2

您可以使用 IFNULL、CASE WHEN 和 UNION 编写更好的解决方案。

解决方案 - http://sqlfiddle.com/#!9/57c8eb3/21

SELECT fn.name AS name_new, fo.date_updated, fo.id, fo.name,fo.price,
fo.product_code, fn.product_code AS product_code_new, fo.stock, fn.price AS price_new, 
fn.stock AS stock_new, fn.date_updated AS date_updated_new,
(case when (IFNULL(fn.product_code,'X') != 'X' and fo.price != fn.price) 
then 'Level1' 
when IFNULL(fn.product_code,'X') = 'X' then 'Level2'
else 'Level4' end) as Ordering_level
FROM feed_old fo LEFT JOIN feed_new fn 
ON fo.product_code = fn.product_code 
UNION
SELECT fn.name AS name_new, fo.date_updated, fo.id, fo.name,fo.price,
fo.product_code, fn.product_code AS product_code_new, fo.stock, 
fn.price AS price_new, fn.stock AS stock_new, fn.date_updated AS date_updated_new,
(case when (IFNULL(fo.product_code,'X') != 'X' and fo.price != fn.price) 
then 'Level1'  
when IFNULL(fo.product_code,'X') = 'X' then 'Level3'
else 'Level4' end) as Ordering_level
FROM feed_old fo RIGHT JOIN feed_new fn 
ON fn.product_code = fo.product_code 
order by ordering_level

如何从 MySQL 中的比较和空检查指定 ORDER BY？

How to specify an ORDER BY from comparisons and null checks in MySQL?

mysql

sql

select

coalesce

sql-order-by