使用单个查询从MYSQL中的2个表中获取基于优先级的记录

Question

有2个具有相同字段的表

表格

1. product_settings
2. store_product_settings

字段列表

1. product_setting_id

2. products_id

3. setting_constant_name

4. setting_value

如何从“store_product_settings”获取记录，如果在“store_product_settings”中找不到，则从“product_settings”表中获取

注意：没有联合查询

Answer 1

您希望在表上使用左外连接，然后如果store_product_settings.setting_constant_name中的值为null，请使用product_settings中的值。

对于我的例子，我假设如下： 1）product_setting_id是每个表的顺序ID，并且是不相关的 2）products_id和setting_constant_name是搜索中使用的两个关键字段（如“我想为产品Y设置X”） 3）setting_value是您希望商店覆盖全局产品的值

所以我在表格中设置了以下值：

insert into product_settings (product_setting_id, products_id, setting_constant_name, setting_value) values (1,1,'S1','V1');
insert into product_settings (product_setting_id, products_id, setting_constant_name, setting_value) values (2,1,'S2','V2');
insert into product_settings (product_setting_id, products_id, setting_constant_name, setting_value) values (3,2,'S1','V1');
insert into product_settings (product_setting_id, products_id, setting_constant_name, setting_value) values (4,2,'S2','V2');
insert into product_settings (product_setting_id, products_id, setting_constant_name, setting_value) values (5,2,'S3','V3');

insert into store_product_settings (product_setting_id, products_id, setting_constant_name, setting_value) values (1,1,'S2','V2A');
insert into store_product_settings (product_setting_id, products_id, setting_constant_name, setting_value) values (2,2,'S1','V1A');
insert into store_product_settings (product_setting_id, products_id, setting_constant_name, setting_value) values (3,2,'S3','V3A');

首先，一个简单的左外连接来显示如何返回所有内容：

select ps.*, sps.* 
from product_settings ps
left outer join store_product_settings sps
  on ps.products_id = sps.products_id
    and ps.setting_constant_name = sps.setting_constant_name

product_setting_id  products_id  setting_constant_name  setting_value  product_setting_id1  products_id1  setting_constant_name1  setting_value1
        1                 1               S1                  V1
        2                 1               S2                  V2              1                   1                 S2                  V2
        3                 2               S1                  V1              2                   2                 S1                  V1
        4                 2               S2                  V2
        5                 2               S3                  V3              3                   2                 S3                  V3

现在，只选择我们想要使用ifnull在store-value上的列来返回一般值：

select ifnull(sps.setting_value,ps.setting_value) as setting_value
from product_settings ps
  left outer join store_product_settings sps on ps.products_id = sps.products_id
    and ps.setting_constant_name = sps.setting_constant_name

给你这个：

setting_value
    V1
    V2A
    V1A
    V2
    V3A

展开要捕获的其他字段的返回集。