Question

在基于Wordpress订单的系统中，我选择的人在某个日期和地点参加活动。

我还需要查看是否存在任何匹配订单的订单备注，我选择meta_value所在的meta_key ="_wc_acof_2"。

问题是当我将条件添加到我的WHERE子句时，代码停止返回不存在元键/值对的记录（即没有订单备注）。而我仍然需要选择那些记录，只为订单备注返回null或类似记录。

WHERE子句中的问题行是：

AND jn_postmeta_guestnotes.meta_key = "_wc_acof_2"

在我的脑海中，我想写出类似于：

的那一行

AND (jn_postmeta_guestnotes.meta_key = "_wc_acof_2" OR 'there is no matching meta_key so just ignore this condition and return the record anyway')

完整查询：

SELECT 
    CONCAT_WS(" ",wp_postmeta.meta_value,jn_postmeta_lastname.meta_value) AS Name,
    wp_woocommerce_order_itemmeta.meta_value AS Adults,
    jn_postmeta_guestnotes.meta_value AS Notes
 FROM
    wp_postmeta
        LEFT JOIN
    wp_postmeta AS jn_postmeta_lastname ON wp_postmeta.post_id = jn_postmeta_lastname.post_id
        LEFT JOIN
    wp_postmeta AS jn_postmeta_guestnotes ON wp_postmeta.post_id = jn_postmeta_guestnotes.post_id
        LEFT JOIN
    wp_posts ON wp_postmeta.post_id = wp_posts.ID
        LEFT JOIN
    wp_woocommerce_order_items ON wp_woocommerce_order_items.order_id = wp_posts.ID
        LEFT JOIN
    wp_woocommerce_order_itemmeta ON wp_woocommerce_order_itemmeta.order_item_id = wp_woocommerce_order_items.order_item_id
        LEFT JOIN
    wp_woocommerce_order_itemmeta AS jn_woocommerce_order_itemmeta_location ON wp_woocommerce_order_itemmeta.order_item_id = jn_woocommerce_order_itemmeta_location.order_item_id
        LEFT JOIN
    wp_woocommerce_order_itemmeta AS jn_woocommerce_order_itemmeta_date ON wp_woocommerce_order_itemmeta.order_item_id = jn_woocommerce_order_itemmeta_date.order_item_id
 WHERE
    1 = 1
        AND wp_postmeta.meta_key = "_billing_first_name"
        AND jn_postmeta_lastname.meta_key = "_billing_last_name"
        AND jn_postmeta_guestnotes.meta_key = "_wc_acof_2"
        AND wp_woocommerce_order_itemmeta.meta_key = "Adults"
        AND jn_woocommerce_order_itemmeta_location.meta_key = "Booking Type"
        AND jn_woocommerce_order_itemmeta_location.meta_value LIKE "%'.$showlocation.'%"
        AND jn_woocommerce_order_itemmeta_date.meta_key = "Booking Date"
        AND (jn_woocommerce_order_itemmeta_date.meta_value = "'.$showdateformata.'" OR jn_woocommerce_order_itemmeta_date.meta_value = "'.$showdateformatb.'")
        AND wp_posts.post_status = "wc-completed"
 GROUP BY wp_posts.ID

任何建议都非常感谢！

Answer 1

答案是将额外的位放在子选择中，如下所示：

(SELECT wp_postmeta.meta_value FROM wp_postmeta WHERE wp_postmeta.meta_key = "_wc_acof_2" AND wp_postmeta.post_id = wp_posts.ID) AS Notes

Answer 2

将所有过滤条件放在ON上，以便左连接起作用。在WHERE上将删除空值

.....
wp_woocommerce_order_itemmeta AS jn_woocommerce_order_itemmeta_date 
    ON wp_woocommerce_order_itemmeta.order_item_id = jn_woocommerce_order_itemmeta_date.order_item_id
   AND 1 = 1 
   AND wp_postmeta.meta_key = "_billing_first_name" 
   AND jn_postmeta_lastname.meta_key = "_billing_last_name" 
   AND jn_postmeta_guestnotes.meta_key = "_wc_acof_2" 
   AND wp_woocommerce_order_itemmeta.meta_key = "Adults" 
   AND jn_woocommerce_order_itemmeta_location.meta_key = "Booking Type" 
   AND jn_woocommerce_order_itemmeta_location.meta_value LIKE "%'.$showlocation.'%" 
   AND jn_woocommerce_order_itemmeta_date.meta_key = "Booking Date" 
   AND 
   ( 
     jn_woocommerce_order_itemmeta_date.meta_value = "'.$showdateformata.'" 
     OR 
     jn_woocommerce_order_itemmeta_date.meta_value = "'.$showdateformatb.'" 
   ) 
   AND wp_posts.post_status = "wc-completed"

MySQL - 如果键/值对不存在，则忽略WHERE条件

2 个答案: